Ashwagandha: What the Evidence Shows

What ashwagandha is, and what extract you’re actually buying

Ashwagandha is the root of an Indian shrub used in Ayurvedic medicine for ~3,000 years. Modern supplements are standardised extracts of the root or root-and-leaf, with the active compounds typically being withanolides. The market has converged on two branded extracts that account for most of the rigorous research: KSM-66 (root only, 5% withanolides minimum) and Sensoril (root and leaf, 10% withanolides). Generic ‘ashwagandha powder’ products vary widely in withanolide content; some have negligible amounts.

This matters for the published evidence: nearly every positive clinical trial used a standardised extract at a known dose. Generic capsules at the same labelled mg may deliver a fraction of the active compound Tandon 2020.

The cortisol-and-stress evidence

Pratte’s 2014 systematic review of 5 RCTs in stressed adults found significant reductions in perceived stress and cortisol after 8-12 weeks of 300-600 mg/day standardised extract Pratte 2014. Lopresti 2019 ran a 60-day RCT in 60 chronically stressed adults and saw 17-23% reductions in morning cortisol and significant improvements in PSS-10 (perceived stress scale) scores compared with placebo Lopresti 2019. Salve 2019’s 8-week RCT showed similar magnitudes Salve 2019. The pattern across this literature:

• Effective doses: 300-600 mg/day of a standardised extract (KSM-66 or Sensoril). Higher doses don’t reliably produce more benefit.
• Time to effect: 4-8 weeks. Reports of ‘next-day’ effects are likely placebo.
• Effect size: cortisol drops typically 15-25%; perceived stress scores drop 30-44% (which sounds large but reflects the high baseline stress in the studied populations).
• Population studied: most trials selected adults with elevated baseline stress. Effects in low-stress populations are smaller and less consistent.

“Ashwagandha root extract demonstrated modest but consistent reductions in serum cortisol and validated stress scores in chronically stressed adults across 5 RCTs. Effect sizes were larger in higher-baseline-stress populations and required 4-12 weeks of daily supplementation.”

— Pratte et al., J Altern Complement Med, 2014 view source

Strength and recovery: where the marketing is ahead of the data

The strength claim originates largely from Wankhede 2015’s 8-week RCT in 57 untrained young men. The ashwagandha group (600 mg/day) gained significantly more strength on bench press and leg extension, and showed greater muscle-circumference increases, than placebo Wankhede 2015. The result was real but limited: the subjects were untrained (where any intervention produces gains), the sample was small (n=57), the trial was funded by a supplement manufacturer, and replication is sparse.

Ziegenfuss 2018’s trial in resistance-trained men found smaller, less reliable effects on strength — closer to the placebo response Ziegenfuss 2018. The 2021 systematic review by Pérez-Gómez and colleagues concluded that ashwagandha’s effects on strength and power are inconsistent and modest at best in trained populations Pérez-Gómez 2020. The supplement is not in the same category as creatine, caffeine, or beta-alanine for strength benefit.

Sleep evidence

The Sanskrit name ashwagandha derives in part from a word that means “sleep-inducing.” Modern trials support this modestly:

• Salve 2020 ran a 6-week RCT in 60 healthy adults and saw improvements in subjective sleep quality (PSQI scores) at 600 mg/day Salve 2020.
• Cheah 2021 meta-analysed 5 RCTs and found small-to-moderate improvements in self-reported sleep quality but limited objective polysomnography evidence Cheah 2021.
• The effect is most reliable in adults with baseline insomnia or significant stress; in already-good sleepers, the additional benefit is small.

Who ashwagandha actually fits

Safety profile: not entirely benign

The published safety profile in healthy adults is generally good at standard doses (300-600 mg/day for ≤12 weeks). But three concerns deserve mention:

• Liver injury: A growing case series in Hepatology Communications and other journals documents drug-induced liver injury (DILI) attributed to ashwagandha in genetically susceptible individuals. Symptoms (jaundice, fatigue, dark urine) typically resolved on discontinuation but required monitoring Bjornsson 2020. Stop and seek medical evaluation if these appear.
• Thyroid effects: ashwagandha can shift thyroid hormones — useful in subclinical hypothyroidism Sharma 2018, problematic in adults already on thyroid medication or with autoimmune thyroid disease.
• Sedation and interaction: at higher doses, mild sedation can occur. Combining with alcohol, benzodiazepines, sleep medications, or central nervous system depressants is unwise.

If you decide to try it

• Choose a standardised extract: KSM-66 or Sensoril. Generic powders without standardised withanolide content are unreliable.
• Dose 300-600 mg/day, divided morning and evening or single morning dose. Higher doses haven’t shown more benefit.
• Take with food: improves absorption and reduces GI complaints.
• Run an 8-12 week trial: most published benefits emerge in this window. Track stress (PSS-10), sleep quality, and one objective marker (resting heart rate, HRV if you have a device).
• Take a break every 12 weeks: limited long-term data; rotation prevents the unknown.
• Stop and consult a physician if: jaundice, dark urine, severe fatigue, persistent rash, mood changes, or thyroid symptoms appear.

What the evidence does NOT support

• “Ashwagandha replaces sleep”: false. The sleep effects are modest improvements in quality, not reduced need for sleep.
• “Ashwagandha is a strength supplement”: insufficient evidence. Effects in trained populations are small and inconsistent.
• “Ashwagandha cures anxiety”: not the same as a treatment for diagnosable anxiety disorder. Modest stress reduction in healthy adults is not a substitute for evidence-based mental health care.
• “Ashwagandha lowers cortisol therefore is anti-aging”: the cortisol-aging chain is far more complex than wellness marketing suggests.
• “Ashwagandha boosts testosterone”: a few small trials show modest testosterone increases in men with reduced baseline levels; effects in eugonadal men are inconsistent and clinically negligible.

Practical takeaways

• Ashwagandha is one of the better-evidenced adaptogens: 5+ RCTs show 15-25% cortisol reduction and meaningful stress-score improvements at 300-600 mg/day for 8-12 weeks.
• Choose a standardised extract (KSM-66 or Sensoril) — generic powders are inconsistent.
• The strength and recovery claims are weaker than marketing suggests; trained athletes should expect modest at best.
• Sleep effects are real but modest; most pronounced in stressed adults with sub-clinical insomnia.
• Safety: thyroid medication interaction, autoimmune contraindication, pregnancy contraindication, recent liver injury reports. Healthy adults at standard doses generally tolerate well.
• Fix sleep, training, and nutrition first. Ashwagandha is a small lever, not a foundation.

References