Ashwagandha (Withania somnifera) has become one of the most popular supplements in the world, driven by claims ranging from stress reduction and better sleep to testosterone boosting and muscle building. It is also the most extensively studied adaptogen, with over 100 human clinical trials. But popularity and evidence are different things. Here is what the research actually supports, what it does not, and how to navigate the two major extract forms.

KSM-66 vs Sensoril: the two extracts that matter

Most clinical research on ashwagandha uses one of two standardized extracts, and they are not interchangeable.

KSM-66 is a full-spectrum root extract standardized to at least 5% withanolides. It is the most widely studied form, with over 24 published clinical trials as of 2025. The standard clinical dose is 300-600mg daily. KSM-66 is produced through a proprietary extraction process that uses no alcohol or chemical solvents, preserving a broader range of bioactive compounds.

Sensoril is extracted from both the root and leaf, standardized to at least 10% withanolides by weight. Because of the higher withanolide concentration, effective doses are lower -- typically 125-250mg daily. Sensoril tends to appear in studies focused on stress, cortisol, and cognitive function.

The distinction matters because a product listing "ashwagandha 600mg" without specifying the extract form could contain generic root powder with vastly different withanolide content and bioavailability than either clinical-grade extract. If the label does not specify KSM-66, Sensoril, or another standardized extract with a defined withanolide percentage, you are largely guessing at what you are getting.

Cortisol and stress: the strongest evidence

This is where ashwagandha's evidence is most compelling. A 2022 systematic review and meta-analysis published in the Journal of Clinical Medicine pooled data from 12 RCTs and found that ashwagandha supplementation significantly reduced serum cortisol levels compared to placebo, with a weighted mean reduction of approximately 11-15%. The effect was dose-dependent and most pronounced in individuals reporting high baseline stress levels.

A landmark 2012 RCT published in the Indian Journal of Psychological Medicine gave 300mg of KSM-66 twice daily (600mg total) to adults with chronic stress for 60 days. The ashwagandha group showed a 27.9% reduction in serum cortisol compared to a 7.9% reduction in the placebo group. Scores on standardized stress assessments (the Perceived Stress Scale and the Depression Anxiety Stress Scale) also improved significantly.

A 2019 study in Medicine using Sensoril at 125mg and 250mg daily found significant reductions in the Hamilton Anxiety Scale scores at both doses after 6 weeks, with the higher dose showing greater benefit. Cortisol reductions were measurable at 250mg but not consistently at 125mg.

The pattern across studies is consistent: ashwagandha at clinical doses reduces cortisol by a modest but meaningful amount and improves self-reported stress and anxiety scores. This is genuine, reproducible, and clinically relevant. However, it is an adaptogenic effect, not a pharmaceutical one. Ashwagandha is not a replacement for clinical treatment of anxiety disorders or chronic stress conditions.

Anxiety: moderate but real evidence

Closely related to the cortisol data, ashwagandha shows moderate evidence for reducing anxiety symptoms. A 2014 systematic review in the Journal of Alternative and Complementary Medicine examined five RCTs and concluded that ashwagandha "significantly improved outcomes on anxiety or stress scales" relative to placebo.

A 2021 meta-analysis in the Journal of Ethnopharmacology was more granular, finding that the anxiolytic effect was significant for generalized anxiety measures but smaller and less consistent for specific anxiety subtypes. The authors noted that study quality was moderate overall and called for larger, longer-duration trials.

For mild to moderate stress-related anxiety, ashwagandha at 300-600mg (KSM-66) or 125-250mg (Sensoril) daily appears to provide meaningful relief. For clinical anxiety disorders, the evidence is insufficient to recommend ashwagandha as a primary intervention, though it may have value as an adjunct. This is a supplement with real effects that fall well short of pharmaceutical efficacy -- which is a perfectly reasonable role for a supplement to fill.

Testosterone: the claim that sells bottles

This is where marketing and science diverge most sharply. Ashwagandha is aggressively marketed as a testosterone booster, with some brands featuring muscular men and implying significant hormonal enhancement. The reality is far more modest.

A 2019 systematic review in the American Journal of Men's Health examined the available evidence and found that ashwagandha supplementation was associated with a statistically significant but modest increase in testosterone levels -- typically 10-22% in men with low baseline levels or high stress. In healthy young men with normal testosterone, the increases were smaller and sometimes not statistically significant.

A 2022 meta-analysis in the journal Health Science Reports pooled 4 RCTs and found that ashwagandha supplementation increased testosterone by an average of approximately 14.7% compared to placebo. Crucially, most participants in these studies were either stressed, infertile, or older men with suboptimal testosterone. The effect appears to be primarily driven by cortisol reduction -- chronically elevated cortisol suppresses testosterone production, so lowering cortisol indirectly allows testosterone to normalize.

What this means practically: if you are a stressed, sleep-deprived man with suboptimal testosterone, ashwagandha may provide a modest boost by reducing the cortisol that is suppressing your levels. If you are a healthy young man with normal testosterone looking for significant enhancement, the evidence does not support meaningful gains. This is cortisol management, not testosterone supplementation.

Sleep quality: emerging evidence

Several recent studies suggest ashwagandha may improve sleep quality, particularly in individuals with stress-related sleep disruption. A 2020 study in the Journal of Ethnopharmacology found that KSM-66 at 600mg daily significantly improved sleep quality scores and sleep onset latency compared to placebo in adults with insomnia. A 2021 RCT reported similar findings with 300mg of KSM-66 twice daily.

The sleep evidence is emerging and promising but not as robust as the stress and cortisol data. The mechanism likely involves the same cortisol-lowering pathway -- lower evening cortisol facilitates better sleep onset and quality.

Safety and side effects

Ashwagandha has a generally favorable safety profile in clinical trials at standard doses for durations up to 12 weeks. The most common side effects are mild GI discomfort (nausea, loose stools) reported in roughly 5-10% of study participants.

However, several important cautions exist. Ashwagandha may interact with thyroid medications -- it has been shown to increase thyroid hormone levels in some studies, which could be problematic for people on thyroid replacement therapy. It should be avoided by pregnant women due to insufficient safety data and traditional contraindications. People on immunosuppressants should exercise caution as ashwagandha may have immunostimulatory effects.

There have also been case reports of liver injury associated with ashwagandha supplements, though these are rare and sometimes involve products with questionable manufacturing quality. Purchasing from brands that use third-party tested, standardized extracts mitigates this risk.

Our take

Ashwagandha is a genuinely effective supplement for stress and cortisol management, with moderate evidence supporting anxiety reduction and emerging evidence for sleep quality. The testosterone claims are real but modest and primarily relevant to stressed or older men with suboptimal levels. It is not a miracle adaptogen, but it is one of the very few herbal supplements with a meaningful evidence base.

Choose KSM-66 at 300-600mg daily or Sensoril at 125-250mg daily. Avoid generic ashwagandha powder without standardization. Expect modest improvements in stress resilience and subjective wellbeing over 4-8 weeks, not dramatic overnight transformation.