Ashwagandha
Adaptogen / HerbAlso known as: Withania somnifera · Indian ginseng · KSM-66 · Sensoril
Ashwagandha (Withania somnifera) is a root herb used for centuries in Ayurvedic medicine, classified as an adaptogen for its proposed ability to help the body resist stressors. Its primary bioactive compounds — withanolides — are thought to modulate the hypothalamic-pituitary-adrenal (HPA) axis. Clinical trials using standardized extracts have found evidence for reductions in perceived stress, cortisol, and anxiety, as well as improvements in sleep quality. Some trials also suggest modest support for testosterone and strength in men. Evidence quality is moderate, with most trials being small and short-term; cycling use is recommended given limited long-term safety data.
Evidence Summary
All 3 studiesAshwagandha (Withania somnifera) is a root herb from Ayurvedic medicine, classified as an adaptogen — a category describing botanicals proposed to help the body resist physical and psychological stressors. Its primary bioactive constituents are withanolides, a class of steroidal lactones that appear to modulate the hypothalamic-pituitary-adrenal (HPA) axis, the central hormonal stress response system. The strongest evidence for ashwagandha relates to stress reduction and cortisol lowering. A well-designed double-blind RCT (Chandrasekhar et al., 2012, n=64) found that a standardized ashwagandha root extract (300 mg twice daily for 60 days) significantly reduced Perceived Stress Scale scores, serum cortisol, and self-reported anxiety and stress compared to placebo. Effect sizes were clinically meaningful in this population. Similar findings have been reported in other RCTs using standardized extracts such as KSM-66 and Sensoril, though trial sizes remain modest (n=40–100 range) and most are of relatively short duration (8–12 weeks). Sleep quality is an emerging area of evidence. A 2019 RCT (Langade et al., n=60) found ashwagandha (300 mg twice daily, KSM-66 extract, 10 weeks) significantly improved sleep onset latency, total sleep time, and sleep efficiency versus placebo, with secondary improvements in anxiety. This aligns with the herb's observed mild sedating quality, which may also explain the preference among practitioners for evening dosing. Testosterone and strength outcomes in men have some RCT support. Wankhede et al. (2015, n=57) found greater 1-RM strength gains, a modest testosterone increase, and reduced post-exercise creatine kinase in men taking KSM-66 alongside resistance training. These findings require replication and should be interpreted with caution — the testosterone effect was statistically significant but modest in absolute terms, and the study was conducted exclusively in males. Several important caveats apply across the ashwagandha evidence base. Most trials use proprietary standardized extracts (KSM-66, Sensoril) with defined withanolide content — results may not generalize to non-standardized powders or preparations with different phytochemical profiles. Industry funding is present in multiple trials, which warrants independent replication. Additionally, there is limited long-term safety data (beyond 3 months), and case reports have raised questions about potential thyroid hormone modulation, though the clinical significance in euthyroid individuals is unclear. Some practitioners recommend cycling (approximately 8–12 weeks on, followed by a 4-week break) as a precautionary approach. Ashwagandha is not established as safe in pregnancy — it was traditionally used as an abortifacient in some systems, and human safety data in pregnant women is absent. It should be avoided during pregnancy.
Read full evidence summary →Top studies
A prospective, randomized double-blind, placebo-controlled study of safety and efficacy of a high-concentration full-spectrum extract of ashwagandha root in reducing stress and anxiety in adults
High-concentration full-spectrum ashwagandha root extract (300 mg twice daily for 60 days) significantly reduced PSS scores, serum cortisol levels, and scores on the anxiety and stress subscales of DASS-21 compared to placebo. No serious adverse events were reported.
May reduce perceived stress, anxiety, and serum cortisol in adults with chronic stress
Well-tolerated in this trial; mild GI complaints in a small number of participants
Relatively small n=64; single study center; used full-spectrum extract (KSM-66 type) — not directly generalizable to all ashwagandha products
Efficacy and Safety of Ashwagandha (Withania somnifera) Root Extract in Insomnia and Anxiety: A Double-blind, Randomized, Placebo-controlled Study
Ashwagandha root extract (KSM-66, 300 mg twice daily for 10 weeks) significantly improved sleep onset latency, total sleep time, sleep efficiency, and morning alertness compared to placebo. Anxiety scores were also significantly lower in the treatment group.
May improve sleep onset, sleep duration, and sleep quality, with secondary benefit on anxiety
No serious adverse events; well-tolerated across both healthy volunteers and insomnia patients
Small n=60; relatively short duration (10 weeks); single standardized extract form used — generalizability to other preparations is uncertain
Expert Mentions
All 3 mentions"Ashwagandha has some decent evidence behind it for reducing cortisol — the stress hormone. I've used it myself and I tend to take it in the evening because it has a somewhat sedating quality. The studies generally support a cortisol-lowering effect, though the magnitude varies."
Huberman describes ashwagandha as one of the more evidence-supported adaptogens for reducing cortisol and stress, and notes it can have a sedating quality that makes it better suited to evening use.
Huberman's characterization of ashwagandha having decent evidence for cortisol reduction is broadly accurate — the Chandrasekhar 2012 RCT and others show significant reductions in serum cortisol and perceived stress. His observation about sedating quality is consistent with the sleep trial data (Langade 2019). However, the phrase "decent evidence" appropriately hedges the moderate quality of individual trials, which are small and often use standardized extracts not representative of all products.
"I'm not dismissive of ashwagandha — there are some reasonable trials showing cortisol reduction and stress effects. But I hold it at arm's length. The studies tend to be small, often industry-sponsored, and we don't have great long-term safety data. If someone is going to use it, I'd suggest cycling — maybe 8 to 12 weeks on, then a break."
Peter Attia expresses cautious interest in ashwagandha but notes the evidence base is modest, the trials are often small and funded by manufacturers, and he recommends cycling it rather than continuous long-term use.
Attia's caution is well-calibrated. Existing RCTs are predominantly small (n=50–100), relatively short (8–12 weeks), and in several cases conducted with manufacturer-provided extracts. Long-term (>6 months) safety data in humans is limited. Cycling is a reasonable precautionary approach that many practitioners advocate, particularly given limited data on thyroid hormone interactions with prolonged use.
Key findings
- ·Double-blind RCT (n=64) found significant reductions in perceived stress, anxiety, and serum cortisol at 300 mg twice daily for 60 days.
- ·RCT (n=60) supports improvements in sleep onset, total sleep time, and sleep efficiency with standardized KSM-66 extract over 10 weeks.
- ·RCT in men (n=57) found modest testosterone increase and greater strength gains when combined with resistance training.
Evidence gaps
- ·Long-term safety data (beyond 12 weeks) is limited; no large longitudinal studies exist.
- ·Most trials are small (n<100), industry-sponsored, and of short duration — independent replication is needed.
- ·Thyroid hormone interaction is a theoretical concern with limited clinical data; euthyroid individuals may not be affected at typical doses.