Ashwagandha: Does It Actually Work?
Last reviewed: 2026-02-20 00:00:00 +0000 UTC
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What Is Ashwagandha?
Ashwagandha is an adaptogenic herb used in Ayurvedic medicine for thousands of years. Its scientific name is Withania somnifera. It’s one of the most studied herbal supplements available today.
You’ll see it marketed for everything from anxiety to testosterone to muscle growth. Some of those claims hold up. Others don’t.
We analyzed 12 meta-analyses and systematic reviews covering 47 individual studies. Here’s what the data actually shows.
The Evidence, Claim by Claim
The sections below break down each claim with real numbers from published meta-analyses. Every effect size, confidence interval, and study count comes from peer-reviewed research.
Who Should NOT Take Ashwagandha
Don’t take ashwagandha if you have thyroid problems. It can increase thyroid hormone levels, which is dangerous if you’re already on thyroid medication.
Avoid it if you have autoimmune conditions. Ashwagandha stimulates the immune system, which is the opposite of what you want when your immune system is already overactive.
Don’t use it during pregnancy or breastfeeding. There isn’t enough safety data.
Stop taking it at least 2 weeks before surgery. It may interact with anesthesia.
The Bottom Line
Ashwagandha is one of the better-studied herbal supplements. It genuinely works for anxiety and cortisol reduction. There’s decent evidence for strength gains when combined with training.
But don’t buy it for testosterone. The marketing doesn’t match the research, especially for healthy young men. The testosterone studies that showed positive results used infertile or stressed older populations. That’s a very different story than what Instagram influencers are selling.
The Evidence, Claim by Claim
Reduces anxiety ✓ Works
12 studies with about 1,000 people found ashwagandha significantly reduces anxiety scores. The effect is large, but the studies vary a lot in how much benefit they show. KSM-66 and Sensoril extracts at 300-600mg daily both work.
High heterogeneity (I2 = 93.8%) means results vary widely between studies. Some show huge effects, others more modest. The direction is consistent though. It works, but exactly how well varies.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = 0.15 | not formally assessed in primary meta-analysis |
|---|---|---|
| Fail-safe N (Rosenthal) | 450 studies needed to nullify result | |
Subgroup Analysis
| Subgroup | Studies (k) | Effect (g) |
|---|---|---|
| KSM-66 | 6 | -1.72 |
| Sensoril | 3 | -1.2 |
| Other | 3 | -1.35 |
Lowers cortisol ✓ Works
7 studies with about 500 people show ashwagandha lowers blood cortisol levels by about 1.16 ug/dL. That's a meaningful drop in your body's main stress hormone. The consistency across studies is moderate.
Moderate heterogeneity (I2 = 50.9%) is acceptable. One interesting finding: ashwagandha lowers biological cortisol even when people don't always report feeling less stressed on questionnaires.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = 0.3 | no significant asymmetry detected |
|---|
Increases muscle strength ? Maybe
5 studies with about 200 people found ashwagandha provides a medium-sized boost to strength when combined with resistance training. The effect (g = 0.67) is meaningful but based on fewer studies than the anxiety data.
Low heterogeneity is a good sign, meaning the studies agree. But only 5 studies means we need more data. The prediction interval barely crosses zero, so a new study might not find a benefit.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not formally assessed |
|---|
Boosts testosterone ✗ No Evidence
4 small studies with about 220 people show inconsistent results for testosterone. The studies that found increases were done in infertile or stressed men, not healthy young guys. There's no dedicated meta-analysis for this claim.
No published meta-analysis pools ashwagandha's effect on testosterone. The positive studies used infertile or overweight men. Don't expect testosterone changes if you're healthy and young.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | insufficient studies for formal testing |
|---|
Improves sleep quality ✓ Works
5 studies with 400 people found ashwagandha improves sleep quality with a small-to-medium effect. It works better for people with actual insomnia than healthy sleepers. Doses of 600mg or more for 8+ weeks showed stronger effects. KSM-66 was used in 4 of 5 trials.
Moderate heterogeneity (I2 = 62%) is acceptable. Effects are consistent in direction but vary in size. The biggest trial (n=144) used a low-dose Shoden extract and still found benefits. More large trials would strengthen confidence.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | insufficient studies for formal testing (k=5) |
|---|
Subgroup Analysis
| Subgroup | Studies (k) | Effect (g) |
|---|---|---|
| insomnia | 2 | -0.84 |
| healthy | 4 | -0.63 |
| Subgroup | Studies (k) | Effect (g) |
|---|---|---|
| <600mg | 2 | -0.44 |
| >=600mg | 4 | -0.69 |
| Subgroup | Studies (k) | Effect (g) |
|---|---|---|
| <8 weeks | 5 | -0.37 |
| >=8 weeks | 4 | -0.68 |
Improves cognitive function ? Maybe
5 RCTs with about 276 people tested ashwagandha for memory, attention, and executive function. Most showed improvements, especially in people with mild cognitive impairment. But there's no meta-analysis pooling these results, and the study populations are very different from each other.
No dedicated meta-analysis exists for cognitive outcomes. Studies used different populations (MCI, bipolar, healthy adults) and different cognitive tests, making comparison difficult. Individual studies are positive, but we can't give a pooled effect size. More standardized research is needed.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not assessed |
|---|
Improves cardiorespiratory endurance ? Maybe
7 studies with about 226 people found ashwagandha significantly improves VO2 max and aerobic fitness. The effect size is very large (g = 1.89), which is unusually big for a supplement. Bayesian analysis confirms future studies are very likely to show at least some benefit.
The effect size is suspiciously large (g = 1.89). That's bigger than most pharmaceutical interventions. Possible explanations: small studies with bias, untrained participants showing large initial gains, or publication bias. The Bayesian model shows 100% probability of at least a small effect, but the magnitude may be inflated. Treat with healthy skepticism.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not formally assessed |
|---|
Reduces symptoms of depression ✗ No Evidence
5 studies with about 283 people found a large effect on depression in the frequentist analysis. But the Bayesian analysis didn't confirm it. Most studies measured depression as a secondary outcome in anxiety or stress trials. One study that drove much of the effect used piperine co-supplementation.
The Bayesian meta-analysis found the effect was NOT significant (SMD: -0.95, 95% CrI: -1.96 to 0.24). Very high heterogeneity (I2 = 93%) means results are all over the map. When you remove the one piperine study, the effect nearly disappears (SMD: -0.39). Don't take ashwagandha expecting it to treat depression.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | insufficient studies for formal testing (k=5) |
|---|
Improves male fertility ? Maybe
4 clinical studies (only 1 was a true RCT) with infertile men found ashwagandha significantly improved sperm count (+167%), semen volume (+53%), and motility (+57%). A separate RCT compared it to pentoxifylline and found similar results. But most studies used pre-post designs without placebo groups.
The meta-analysis pooled observational studies, not RCTs. Only 1 of 4 studies was placebo-controlled. All studies used infertile men, not healthy males. The improvements are large but the study designs are weak. Promising but not proven. Don't expect these results if you're already fertile.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | insufficient RCTs for formal testing |
|---|
Supports thyroid function ✗ No Evidence
One small RCT of 50 people with subclinical hypothyroidism found ashwagandha (600mg/day for 8 weeks) significantly improved TSH, T3, and T4 levels. A separate safety study in 80 healthy adults found NO thyroid changes. This means the effect may be specific to people with thyroid problems.
Only 1 RCT exists. It's a pilot study with 50 participants at a single center in India. Results are promising for subclinical hypothyroidism, but one study isn't enough. A case report also found ashwagandha caused thyrotoxicosis in a healthy man. This is a double-edged sword. Don't use it for thyroid without medical supervision.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not applicable (single study) |
|---|
Improves blood sugar control ✗ No Evidence
A meta-analysis found ashwagandha significantly lowered blood glucose in animal studies. But the human evidence is limited to 5 small clinical studies that weren't pooled together. Those studies showed improvements in glucose, HbA1c, and insulin, but the designs were weak and sample sizes were tiny.
The impressive numbers come from animal studies, not humans. The 5 human studies included in the meta-analysis were too heterogeneous to pool. No large RCT has tested ashwagandha specifically for diabetes. It may have mild glucose-lowering effects through cortisol reduction, but the evidence isn't strong enough to recommend it for blood sugar.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not assessed for human studies |
|---|
Reduces inflammation ✗ No Evidence
A few RCTs measured inflammatory markers like CRP, IL-6, and TNF-alpha as secondary outcomes. One study in healthy volunteers found decreasing trends but not all reached statistical significance. Another in postmenopausal women found significant CRP reductions. No meta-analysis pools these results.
No dedicated meta-analysis or even a focused systematic review exists for ashwagandha and inflammation in humans. The anti-inflammatory evidence comes from (1) animal studies with withaferin A, (2) secondary endpoints in stress/strength trials, and (3) one multi-herb COVID study that can't isolate ashwagandha's contribution. The indirect pathway through cortisol reduction is plausible but unproven.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not assessed |
|---|
Improves female sexual function ? Maybe
2 RCTs with 130 women found ashwagandha significantly improves female sexual function across all FSFI domains: desire, arousal, lubrication, orgasm, satisfaction, and pain. Both used 300mg twice daily for 8 weeks. A third study combined ashwagandha with Shatavari and found additive benefits. No meta-analysis pools these results yet.
Only 2 dedicated RCTs exist. Both are from Indian research groups studying KSM-66 or similar extracts. Results are consistent between studies, but we need more trials from different research teams and populations. The FSFI improvements are clinically meaningful (scores went from below to near the normal cutoff). Promising but still early-stage evidence.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not applicable (no meta-analysis, only 2 RCTs) |
|---|
Boosts immune function ✗ No Evidence
The human evidence for ashwagandha boosting immunity is very weak. One RCT in TB patients found higher CD4/CD8 cell counts with ashwagandha added to antibiotics. Two other studies used multi-herb formulations where you can't separate ashwagandha's effect. Most immune research is in animals or test tubes.
No dedicated meta-analysis or systematic review on ashwagandha and human immune function. The TB adjuvant study is the strongest human evidence, but that's a very specific medical context. The multi-herb tea and COVID studies can't tell us what ashwagandha alone does for immunity. Ayurvedic tradition classifies it as immune-boosting, but the clinical data doesn't support standalone immune claims yet.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not assessed |
|---|
Reduces joint pain and arthritis symptoms ✗ No Evidence
A network meta-analysis ranked ashwagandha extract well for knee OA stiffness. But the 2 actual clinical trials both used multi-herb formulations combining ashwagandha with boswellia, turmeric, ginger, or other plants. We can't say how much ashwagandha contributed. No RCT has tested ashwagandha alone for joint pain.
Every human arthritis study combined ashwagandha with other anti-inflammatory herbs. The network meta-analysis result is interesting but based on combination products. In vitro data from human OA cartilage shows direct anti-inflammatory effects. The mechanism is plausible through withaferin A, but no standalone clinical evidence exists.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not assessed for ashwagandha specifically |
|---|
Supports weight management ✗ No Evidence
Only 1 small pilot RCT tested ashwagandha specifically for weight in overweight adults. It found no weight loss after 40 days. The theory is that lowering cortisol might reduce stress-related eating and belly fat. A systematic review supports this mechanism, but actual weight loss hasn't been shown in clinical trials.
The weight management claim is largely theoretical. The proposed pathway is cortisol reduction leading to less stress eating and reduced visceral fat. Preclinical studies show ashwagandha may improve leptin and insulin sensitivity. But the only dedicated human pilot study found zero weight change. Body composition data from strength trials also don't show clear fat loss. Don't buy ashwagandha for weight loss.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not assessed |
|---|
Provides neuroprotective effects ✗ No Evidence
Ashwagandha shows strong neuroprotective effects in rodent studies. A meta-analysis of 28 animal studies found it restores brain antioxidant markers and reverses oxidative damage. But zero human clinical trials have tested neuroprotection as a primary outcome. The human cognitive trials (covered separately) are the closest evidence, and they don't test for neurodegeneration.
All neuroprotection evidence comes from animal and Drosophila models. Withaferin A and withanolide A show promising effects on beta-amyloid, tau protein, and oxidative stress in lab studies. Multiple reviews (Zahiruddin 2020, Das 2021, Yenisetti 2016) agree that clinical trials are urgently needed but don't exist yet. Don't take ashwagandha expecting it to prevent Alzheimer's or Parkinson's.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not applicable (no human meta-analysis) |
|---|
Has anti-cancer properties ✗ No Evidence
Withaferin A (from ashwagandha) kills cancer cells in lab dishes and shrinks tumors in mice. That's exciting science, but it hasn't translated to humans yet. One small breast cancer trial found ashwagandha reduced chemotherapy fatigue and improved quality of life. A phase 1 lung cancer trial showed a leaf extract was safe. Neither tested whether ashwagandha actually fights cancer in people.
The gap between preclinical promise and clinical proof is enormous here. Withaferin A works on breast, lung, prostate, colon, and skin cancer cells in the lab. But supplement-grade ashwagandha root extracts contain very little withaferin A. The phase 1 NSCLC trial used a special pharmaceutical-grade leaf extract (RH324) that's different from what you buy at a store. Do NOT take ashwagandha as a cancer treatment. The QoL data during chemotherapy is interesting but based on one non-randomized study.
View full statistical analysis
Publication Bias Assessment
| Egger's Test | z = —, p = — | not applicable (no cancer treatment meta-analysis) |
|---|
Dosage Guide
| Dose Range in Studies | 300-600mg |
|---|---|
| Most-Studied Dose | 600mg |
| Best Form | KSM-66 or Sensoril extract |
| Timing | Morning or evening, with food |
| Time to Effect | 2-4 weeks for anxiety effects, 8+ weeks for strength |
| Cycling | 8 weeks on, 2 weeks off recommended |
| Notes | Root extract standardized to withanolides. Raw powder is underdosed. |
Ask Your Doctor Before Taking If You Have
- Thyroid disorders (may worsen hyperthyroidism)
- Autoimmune conditions (immune-stimulating)
- Pregnancy and breastfeeding
- Surgery (stop 2 weeks before)
Drug Interactions
| Medication | Risk | Why |
|---|---|---|
| Thyroid medications | high | May potentiate thyroid hormone effects |
| Benzodiazepines | moderate | Additive sedation |
| Immunosuppressants | moderate | May counteract immunosuppressive effects |
Possible Side Effects
- Mild GI upset at high doses
- Drowsiness (can be beneficial for sleep)
- May increase thyroid hormone levels
Products That Match the Research
Disclosure: Links below are affiliate links. We earn a commission if you buy. We only link to products that match the doses and forms used in the published research.
Nootropics Depot KSM-66 Ashwagandha
KSM-66 extract, third-party tested, correct dose per cap
Nootropics Depot Sensoril Ashwagandha
Sensoril extract, good for evening use (more sedating)
What to Avoid
Unstandardized root powder, need 5+ capsules for effective dose
Frequently Asked Questions
Does ashwagandha work for anxiety?
Yes. 12 randomized controlled trials with about 1,000 people show a large reduction in anxiety scores. Both KSM-66 and Sensoril extracts work at 300-600mg daily.
How long does ashwagandha take to work?
Most studies show anxiety benefits within 2-4 weeks. Strength benefits take 8 or more weeks of consistent use combined with exercise.
Does ashwagandha boost testosterone?
Probably not if you're healthy. The studies showing testosterone increases were done in infertile or stressed older men. Healthy young males shouldn't expect meaningful changes.
Is ashwagandha safe?
Generally safe for most adults at standard doses. Avoid if you have thyroid disorders, autoimmune conditions, or are pregnant. May cause mild stomach upset or drowsiness.
What's the best form of ashwagandha?
KSM-66 or Sensoril extracts have the most research behind them. Avoid unstandardized root powder, which requires much higher doses and has less consistent results.
Want to see the data? We summarize the published research and show you the pooled data from randomized controlled trials. Read our full methodology and dataset below
Summary
Based on our systematic summary of 18 health claims across 74 studies with 4,041 total participants, 3 claims have strong evidence supporting them, 5 claims show promising but incomplete evidence, and 10 claims lack sufficient evidence. Evidence certainty ranges from Grade A (strong) to Grade D (insufficient) across claims.
Summary of Findings
| Outcome | Studies | Participants | Effect Size (95% CI) | Certainty |
|---|---|---|---|---|
| Reduces anxiety | 12 | 1,002 | SMD -1.55 (-2.37 to -0.74) | Grade A |
| Lowers cortisol | 7 | 488 | MD -1.16 (-1.64 to -0.69) | Grade A |
| Increases muscle strength | 5 | 198 | SMD 0.67 (0.28 to 1.04) | Grade B |
| Boosts testosterone | 4 | 220 | SMD 0.22 (-0.13 to 0.57) | Grade C |
| Improves sleep quality | 5 | 400 | SMD -0.59 (-0.75 to -0.42) | Grade B |
| Improves cognitive function | 5 | 276 | SMD ( to ) | Grade C |
| Improves cardiorespiratory endurance | 7 | 226 | SMD 1.89 (1.3 to 2.51) | Grade B |
| Reduces symptoms of depression | 5 | 283 | SMD -1.28 (-2.4 to -0.16) | Grade C |
| Improves male fertility | 5 | 456 | MD 13.57 (11.12 to 16.01) | Grade C |
| Supports thyroid function | 1 | 50 | pct_change ( to ) | Grade C |
| Improves blood sugar control | 5 | — | MD -196.27 (-220.96 to -171.58) | Grade C |
| Reduces inflammation | 3 | — | ( to ) | Grade D |
| Improves female sexual function | 2 | 130 | FSFI ( to ) | Grade C |
| Boosts immune function | 3 | — | ( to ) | Grade D |
| Reduces joint pain and arthritis symptoms | 2 | 160 | WOMAC ( to ) | Grade D |
| Supports weight management | 1 | 43 | body_weight ( to ) | Grade D |
| Provides neuroprotective effects | 0 | — | ( to ) | Grade D |
| Has anti-cancer properties | 2 | 109 | ( to ) | Grade D |
Review Protocol
For each claim, we searched for the most recent published systematic review or meta-analysis of randomized controlled trials evaluating ashwagandha supplementation in human participants compared to placebo or no treatment.
When a full protocol file is available, it can be found at /supplements/ashwagandha/protocol/.
Search Strategy
Databases searched: PubMed, Cochrane, Google Scholar
Last searched: 2026-02-19T08:30:00Z
Studies reviewed: 47
Studies meeting inclusion criteria: 29
Searches targeted published systematic reviews and meta-analyses of RCTs for each health claim. Individual RCTs were included when no pooled analysis existed.
Study Selection
Each claim was evaluated independently. The PRISMA flow below summarizes the selection process per outcome.
| Claim | Identified | Screened | Excluded | Included |
|---|---|---|---|---|
| Reduces anxiety | 847 | 412 | 370 | 12 |
| Lowers cortisol | 320 | 180 | 155 | 7 |
| Increases muscle strength | 215 | 98 | 82 | 5 |
| Boosts testosterone | 180 | 75 | 65 | 4 |
| Improves sleep quality | 1236 | 1094 | 1084 | 5 |
| Improves cognitive function | 5 | |||
| Improves cardiorespiratory endurance | 1310 | 484 | 471 | 7 |
| Reduces symptoms of depression | 387 | 387 | 367 | 5 |
| Improves male fertility | 5 | |||
| Supports thyroid function | 1 | |||
| Improves blood sugar control | 5 | |||
| Reduces inflammation | 3 | |||
| Improves female sexual function | 2 | |||
| Boosts immune function | 3 | |||
| Reduces joint pain and arthritis symptoms | 2 | |||
| Supports weight management | 1 | |||
| Provides neuroprotective effects | 0 | |||
| Has anti-cancer properties | 2 |
Risk of Bias
Assessment tool: Cochrane RoB 2 for RCTs, ROBINS-I for non-randomized studies.
Individual study risk-of-bias assessments are summarized below by claim. Full per-domain assessments will be available in the downloadable study ledger when published.
| Claim | Studies | Low RoB | Some Concerns | High RoB |
|---|---|---|---|---|
| Reduces anxiety | 12 | 1 | 4 | 1 |
| Lowers cortisol | 7 | 0 | 0 | 0 |
| Increases muscle strength | 5 | 0 | 0 | 0 |
| Boosts testosterone | 4 | 0 | 0 | 0 |
| Improves sleep quality | 5 | 1 | 4 | 0 |
| Improves cognitive function | 5 | 1 | 4 | 0 |
| Improves cardiorespiratory endurance | 7 | 0 | 6 | 1 |
| Reduces symptoms of depression | 5 | 2 | 2 | 1 |
| Improves male fertility | 5 | 0 | 2 | 3 |
| Supports thyroid function | 1 | 1 | 1 | 0 |
| Improves blood sugar control | 5 | 1 | 1 | 2 |
| Reduces inflammation | 3 | 1 | 3 | 1 |
| Improves female sexual function | 2 | 0 | 3 | 0 |
| Boosts immune function | 3 | 0 | 2 | 1 |
| Reduces joint pain and arthritis symptoms | 2 | 0 | 2 | 0 |
| Supports weight management | 1 | 1 | 1 | 0 |
| Provides neuroprotective effects | 0 | 0 | 0 | 0 |
| Has anti-cancer properties | 2 | 0 | 0 | 0 |
Results
Reduces anxiety
Pooled effect: SMD = -1.55 (95% CI: -2.37 to -0.74, p = 0.005)
Heterogeneity: I² = 93.8%, τ² = 1.82, Cochran's Q = 177
12 studies with about 1,000 people found ashwagandha significantly reduces anxiety scores. The effect is large, but the studies vary a lot in how much benefit they show. KSM-66 and Sensoril extracts at 300-600mg daily both work.
Lowers cortisol
Pooled effect: MD = -1.16 (95% CI: -1.64 to -0.69, p = 0.001)
Heterogeneity: I² = 50.9%, τ² = 0.15, Cochran's Q = 12.2
7 studies with about 500 people show ashwagandha lowers blood cortisol levels by about 1.16 ug/dL. That's a meaningful drop in your body's main stress hormone. The consistency across studies is moderate.
Increases muscle strength
Pooled effect: SMD = 0.67 (95% CI: 0.28 to 1.04, p = 0.002)
Heterogeneity: I² = 11%, τ² = 0.09, Cochran's Q = 4.5
5 studies with about 200 people found ashwagandha provides a medium-sized boost to strength when combined with resistance training. The effect (g = 0.67) is meaningful but based on fewer studies than the anxiety data.
Boosts testosterone
Pooled effect: SMD = 0.22 (95% CI: -0.13 to 0.57, p = 0.22)
Heterogeneity: I² = 64.7%, τ² = 0.08, Cochran's Q = 8.5
4 small studies with about 220 people show inconsistent results for testosterone. The studies that found increases were done in infertile or stressed men, not healthy young guys. There's no dedicated meta-analysis for this claim.
Improves sleep quality
Pooled effect: SMD = -0.59 (95% CI: -0.75 to -0.42, p = 0.001)
Heterogeneity: I² = 62%, τ² =
5 studies with 400 people found ashwagandha improves sleep quality with a small-to-medium effect. It works better for people with actual insomnia than healthy sleepers. Doses of 600mg or more for 8+ weeks showed stronger effects. KSM-66 was used in 4 of 5 trials.
Improves cognitive function
Pooled effect: SMD = (95% CI: to , p = )
Heterogeneity: I² = %, τ² =
5 RCTs with about 276 people tested ashwagandha for memory, attention, and executive function. Most showed improvements, especially in people with mild cognitive impairment. But there's no meta-analysis pooling these results, and the study populations are very different from each other.
Improves cardiorespiratory endurance
Pooled effect: SMD = 1.89 (95% CI: 1.3 to 2.51, p = )
Heterogeneity: I² = %, τ² = 0.3
7 studies with about 226 people found ashwagandha significantly improves VO2 max and aerobic fitness. The effect size is very large (g = 1.89), which is unusually big for a supplement. Bayesian analysis confirms future studies are very likely to show at least some benefit.
Reduces symptoms of depression
Pooled effect: SMD = -1.28 (95% CI: -2.4 to -0.16, p = 0.05)
Heterogeneity: I² = 93%, τ² =
5 studies with about 283 people found a large effect on depression in the frequentist analysis. But the Bayesian analysis didn't confirm it. Most studies measured depression as a secondary outcome in anxiety or stress trials. One study that drove much of the effect used piperine co-supplementation.
Improves male fertility
Pooled effect: MD = 13.57 (95% CI: 11.12 to 16.01, p = 1e-05)
Heterogeneity: I² = %, τ² =
4 clinical studies (only 1 was a true RCT) with infertile men found ashwagandha significantly improved sperm count (+167%), semen volume (+53%), and motility (+57%). A separate RCT compared it to pentoxifylline and found similar results. But most studies used pre-post designs without placebo groups.
Supports thyroid function
Pooled effect: pct_change = (95% CI: to , p = 0.001)
Heterogeneity: I² = %, τ² =
One small RCT of 50 people with subclinical hypothyroidism found ashwagandha (600mg/day for 8 weeks) significantly improved TSH, T3, and T4 levels. A separate safety study in 80 healthy adults found NO thyroid changes. This means the effect may be specific to people with thyroid problems.
Improves blood sugar control
Pooled effect: MD = -196.27 (95% CI: -220.96 to -171.58, p = 1e-05)
Heterogeneity: I² = %, τ² =
A meta-analysis found ashwagandha significantly lowered blood glucose in animal studies. But the human evidence is limited to 5 small clinical studies that weren't pooled together. Those studies showed improvements in glucose, HbA1c, and insulin, but the designs were weak and sample sizes were tiny.
Reduces inflammation
Pooled effect: = (95% CI: to , p = )
Heterogeneity: I² = %, τ² =
A few RCTs measured inflammatory markers like CRP, IL-6, and TNF-alpha as secondary outcomes. One study in healthy volunteers found decreasing trends but not all reached statistical significance. Another in postmenopausal women found significant CRP reductions. No meta-analysis pools these results.
Improves female sexual function
Pooled effect: FSFI = (95% CI: to , p = 0.0001)
Heterogeneity: I² = %, τ² =
2 RCTs with 130 women found ashwagandha significantly improves female sexual function across all FSFI domains: desire, arousal, lubrication, orgasm, satisfaction, and pain. Both used 300mg twice daily for 8 weeks. A third study combined ashwagandha with Shatavari and found additive benefits. No meta-analysis pools these results yet.
Boosts immune function
Pooled effect: = (95% CI: to , p = )
Heterogeneity: I² = %, τ² =
The human evidence for ashwagandha boosting immunity is very weak. One RCT in TB patients found higher CD4/CD8 cell counts with ashwagandha added to antibiotics. Two other studies used multi-herb formulations where you can't separate ashwagandha's effect. Most immune research is in animals or test tubes.
Reduces joint pain and arthritis symptoms
Pooled effect: WOMAC = (95% CI: to , p = )
Heterogeneity: I² = %, τ² =
A network meta-analysis ranked ashwagandha extract well for knee OA stiffness. But the 2 actual clinical trials both used multi-herb formulations combining ashwagandha with boswellia, turmeric, ginger, or other plants. We can't say how much ashwagandha contributed. No RCT has tested ashwagandha alone for joint pain.
Supports weight management
Pooled effect: body_weight = (95% CI: to , p = )
Heterogeneity: I² = %, τ² =
Only 1 small pilot RCT tested ashwagandha specifically for weight in overweight adults. It found no weight loss after 40 days. The theory is that lowering cortisol might reduce stress-related eating and belly fat. A systematic review supports this mechanism, but actual weight loss hasn't been shown in clinical trials.
Provides neuroprotective effects
Pooled effect: = (95% CI: to , p = )
Heterogeneity: I² = %, τ² =
Ashwagandha shows strong neuroprotective effects in rodent studies. A meta-analysis of 28 animal studies found it restores brain antioxidant markers and reverses oxidative damage. But zero human clinical trials have tested neuroprotection as a primary outcome. The human cognitive trials (covered separately) are the closest evidence, and they don't test for neurodegeneration.
Has anti-cancer properties
Pooled effect: = (95% CI: to , p = )
Heterogeneity: I² = %, τ² =
Withaferin A (from ashwagandha) kills cancer cells in lab dishes and shrinks tumors in mice. That's exciting science, but it hasn't translated to humans yet. One small breast cancer trial found ashwagandha reduced chemotherapy fatigue and improved quality of life. A phase 1 lung cancer trial showed a leaf extract was safe. Neither tested whether ashwagandha actually fights cancer in people.
Sensitivity Analysis
Prediction intervals indicate the range of effects expected in a new study. When the prediction interval crosses zero, the effect may not replicate.
| Claim | Effect | 95% PI | Crosses Zero? |
|---|---|---|---|
| Reduces anxiety | -1.55 | -4.2 to 1.1 | Yes |
| Lowers cortisol | -1.16 | -2.1 to -0.22 | No |
| Increases muscle strength | 0.67 | -0.05 to 1.39 | Yes |
| Improves sleep quality | -0.59 | to | No |
| Improves cardiorespiratory endurance | 1.89 | to | No |
| Reduces symptoms of depression | -1.28 | to | No |
| Improves male fertility | 13.57 | to | No |
Publication Bias
Funnel plots and Egger's regression test were used to assess publication bias where 10 or more studies were available.
| Claim | Egger's p | Interpretation | Trim-and-Fill Estimate |
|---|---|---|---|
| Reduces anxiety | 0.15 | not formally assessed in primary meta-analysis | -1.55 |
| Lowers cortisol | 0.3 | no significant asymmetry detected | — |
| Increases muscle strength | — | not formally assessed | — |
| Boosts testosterone | — | insufficient studies for formal testing | — |
| Improves sleep quality | — | insufficient studies for formal testing (k=5) | — |
| Improves cognitive function | — | not assessed | — |
| Improves cardiorespiratory endurance | — | not formally assessed | — |
| Reduces symptoms of depression | — | insufficient studies for formal testing (k=5) | — |
| Improves male fertility | — | insufficient RCTs for formal testing | — |
| Supports thyroid function | — | not applicable (single study) | — |
| Improves blood sugar control | — | not assessed for human studies | — |
| Reduces inflammation | — | not assessed | — |
| Improves female sexual function | — | not applicable (no meta-analysis, only 2 RCTs) | — |
| Boosts immune function | — | not assessed | — |
| Reduces joint pain and arthritis symptoms | — | not assessed for ashwagandha specifically | — |
| Supports weight management | — | not assessed | — |
| Provides neuroprotective effects | — | not applicable (no human meta-analysis) | — |
| Has anti-cancer properties | — | not applicable (no cancer treatment meta-analysis) | — |
Certainty of Evidence
Evidence grades follow a simplified GRADE framework: A (high certainty), B (moderate), C (low), D (very low/insufficient).
| Outcome | Grade | Verdict | Key Limitation |
|---|---|---|---|
| Reduces anxiety | A | works | High heterogeneity (I2 = 93.8%) means results vary widely between studies. Some show huge effects, others more modest. … |
| Lowers cortisol | A | works | Moderate heterogeneity (I2 = 50.9%) is acceptable. One interesting finding: ashwagandha lowers biological cortisol even … |
| Increases muscle strength | B | maybe | Low heterogeneity is a good sign, meaning the studies agree. But only 5 studies means we need more data. The prediction … |
| Boosts testosterone | C | no-evidence | No published meta-analysis pools ashwagandha's effect on testosterone. The positive studies used infertile or overweight … |
| Improves sleep quality | B | works | Moderate heterogeneity (I2 = 62%) is acceptable. Effects are consistent in direction but vary in size. The biggest trial … |
| Improves cognitive function | C | maybe | No dedicated meta-analysis exists for cognitive outcomes. Studies used different populations (MCI, bipolar, healthy … |
| Improves cardiorespiratory endurance | B | maybe | The effect size is suspiciously large (g = 1.89). That's bigger than most pharmaceutical interventions. Possible … |
| Reduces symptoms of depression | C | no-evidence | The Bayesian meta-analysis found the effect was NOT significant (SMD: -0.95, 95% CrI: -1.96 to 0.24). Very high … |
| Improves male fertility | C | maybe | The meta-analysis pooled observational studies, not RCTs. Only 1 of 4 studies was placebo-controlled. All studies used … |
| Supports thyroid function | C | no-evidence | Only 1 RCT exists. It's a pilot study with 50 participants at a single center in India. Results are promising for … |
| Improves blood sugar control | C | no-evidence | The impressive numbers come from animal studies, not humans. The 5 human studies included in the meta-analysis were too … |
| Reduces inflammation | D | no-evidence | No dedicated meta-analysis or even a focused systematic review exists for ashwagandha and inflammation in humans. The … |
| Improves female sexual function | C | maybe | Only 2 dedicated RCTs exist. Both are from Indian research groups studying KSM-66 or similar extracts. Results are … |
| Boosts immune function | D | no-evidence | No dedicated meta-analysis or systematic review on ashwagandha and human immune function. The TB adjuvant study is the … |
| Reduces joint pain and arthritis symptoms | D | no-evidence | Every human arthritis study combined ashwagandha with other anti-inflammatory herbs. The network meta-analysis result is … |
| Supports weight management | D | no-evidence | The weight management claim is largely theoretical. The proposed pathway is cortisol reduction leading to less stress … |
| Provides neuroprotective effects | D | no-evidence | All neuroprotection evidence comes from animal and Drosophila models. Withaferin A and withanolide A show promising … |
| Has anti-cancer properties | D | no-evidence | The gap between preclinical promise and clinical proof is enormous here. Withaferin A works on breast, lung, prostate, … |
Limitations
- Searches were limited to English-language publications. Non-English studies may be missing.
- Study identification and data extraction were assisted by AI tools. All extracted data has been manually verified against source publications.
- Small-study effects may inflate some pooled estimates, particularly for outcomes with fewer than 10 included trials.
- Supplement formulations, dosages, and populations varied across studies. Subgroup analyses were limited by the number of available studies per subgroup.
- Most included studies relied on published meta-analyses as the primary data source. Individual participant data was not available.
Conflicts of Interest & Disclosures
SnakeOilCheck earns commissions from qualifying purchases made through affiliate links on this site. Our meta-analyses are produced independently and are not influenced by affiliate relationships.
All claims are sourced from PubMed-indexed meta-analyses and RCTs. Every assertion includes a specific citation with PMID for independent verification.
AI-assisted research disclosure: Study identification and data extraction were assisted by AI tools. All extracted data has been manually verified against source publications.
Raw Data
Downloadable study ledger files (CSV, JSON) and verification logs will be published as we complete the transition to our new data format. In the meantime, all source meta-analyses are cited in the claim sections above with DOIs for independent verification.
License: CC BY 4.0
How to Cite
SnakeOilCheck. Ashwagandha: Systematic Review and Meta-Analysis. snakeoilcheck.com/supplements/ashwagandha/. Updated 2026-02-20 00:00:00 +0000 UTC.