The Ashwagandha Boom
Ashwagandha (Withania somnifera) has gone from an obscure Ayurvedic herb to one of the top-selling supplements in America. Sales have grown by double digits annually for the past several years. It is marketed for everything from stress relief to testosterone boosting to muscle building to thyroid support.
The research base has genuinely expanded in recent years. But as with any popular supplement, the marketing claims have expanded faster than the evidence. Here is what the clinical trials actually show, organized by claim.
Two Extracts, Two Evidence Bases
Before diving into the evidence, you need to understand that "ashwagandha" is not one product. The two main standardized extracts are very different:
KSM-66 is a full-spectrum root extract standardized to at least 5% withanolides. It is the more extensively studied extract, with over 20 published clinical trials. The standard dose is 600 mg per day (usually 300 mg twice daily).
Sensoril is an extract from both roots and leaves, standardized to at least 10% withanolides. It tends to have a more sedating/calming effect and is often used in products targeting sleep and relaxation. The standard dose is 125-250 mg per day.
Generic ashwagandha root powder is unstandandardized, meaning the withanolide content is variable and unpredictable. Most clinical trials did not use generic powder, so the results may not apply to these products. Despite this, generic powder is the cheapest option and is found in many products.
When we say "the evidence shows X," we are always referring to a specific extract at a specific dose. This matters when choosing a product. Our ashwagandha scorecard lists which extract each product uses.
Claim 1: Stress and Anxiety Reduction
Verdict: Strong evidence. This is ashwagandha's strongest use case.
Multiple well-designed RCTs have found that ashwagandha significantly reduces perceived stress and anxiety symptoms. A 2014 systematic review covering five RCTs concluded that ashwagandha substantially improved anxiety scores compared to placebo. A 2020 meta-analysis of five RCTs published in the Journal of Alternative and Complementary Medicine confirmed significant anxiolytic effects.
Key studies include:
- A 2012 RCT (64 participants) using KSM-66 at 600 mg/day found a 28% reduction in serum cortisol levels and significant improvements in all stress-assessment scales compared to placebo
- A 2019 RCT (60 participants) using KSM-66 at 240 mg/day found significant reductions in anxiety scores on the Hamilton Anxiety Rating Scale (HAM-A)
- A 2008 RCT using Sensoril at 125-250 mg/day found significant improvements in anxiety and stress markers, with the higher dose performing better
The cortisol reduction finding is particularly consistent across studies. Cortisol is a stress hormone, and chronically elevated cortisol is associated with anxiety, weight gain, poor sleep, and immune suppression. The 20-30% cortisol reduction seen in multiple trials is a meaningful physiological change.
For stress and anxiety, the evidence quality is genuinely good. The effect sizes are moderate (not transformative, but noticeable), and the results are consistent across different research groups and study designs.
Claim 2: Sleep Quality
Verdict: Moderate evidence. Works primarily through stress reduction.
A 2019 RCT published in Cureus (150 participants) found that ashwagandha (600 mg/day of root extract) significantly improved sleep quality scores and sleep onset latency compared to placebo over 10 weeks. A 2020 meta-analysis confirmed a significant positive effect on sleep quality across available trials.
However, most researchers believe the sleep benefit is secondary to the stress/anxiety reduction. Ashwagandha does not appear to be a direct sedative (unlike melatonin, for example). If your insomnia is driven by a racing mind and elevated stress, ashwagandha may help. If your sleep issue is purely circadian (jet lag, shift work), melatonin or light therapy would be more appropriate.
Sensoril may have a slight edge over KSM-66 for sleep specifically, based on its calming profile, but direct head-to-head comparisons are limited.
Claim 3: Testosterone and Male Reproductive Health
Verdict: Promising but overhyped. Context matters enormously.
This is where the marketing has outpaced the science most dramatically. Several studies have found that ashwagandha supplementation increases testosterone levels, but the details matter:
- A 2010 study found that ashwagandha improved semen parameters and testosterone levels in infertile men (not healthy men with normal testosterone)
- A 2015 study in young men doing resistance training found a significant increase in testosterone with KSM-66 compared to placebo, but the baseline testosterone levels were in the normal range, and the increase was modest
- A 2019 RCT in overweight men aged 40-70 found a 14.7% increase in testosterone with KSM-66 at 600 mg/day over 8 weeks
Here is the nuance that most marketing materials skip: the testosterone increases seen in studies tend to be in the range of 10-15%, and they appear most pronounced in stressed men, overweight men, or men with suboptimal baseline levels. For a healthy, non-stressed young man with normal testosterone, the effect may be clinically insignificant.
Ashwagandha is not a steroid. It will not produce dramatic muscle-building or body-composition changes through testosterone alone. The testosterone increases are real but modest, and likely mediated through cortisol reduction (chronically elevated cortisol suppresses testosterone production).
Claim 4: Athletic Performance and Muscle Building
Verdict: Moderate evidence for modest benefits.
Several RCTs have tested ashwagandha in exercise contexts:
- A 2015 study found that KSM-66 (600 mg/day) significantly improved muscle strength and recovery compared to placebo in young men performing resistance training over 8 weeks
- A 2012 pilot study found improved cardiorespiratory endurance in healthy athletic adults taking ashwagandha root extract
- A 2021 systematic review of 12 studies found that ashwagandha supplementation had a small but significant positive effect on VO2max, strength, and recovery
The effects are real but modest. Ashwagandha is not creatine. The performance benefits are small and may be partly explained by improved recovery (less cortisol, better sleep) rather than direct ergogenic effects. If you are already taking creatine, a protein supplement, and sleeping well, adding ashwagandha to your training stack will produce marginal additional benefits at best.
Claim 5: Thyroid Function
Verdict: Very limited evidence. Proceed with caution.
A 2018 study in the Journal of Alternative and Complementary Medicine found that ashwagandha (600 mg/day of root extract) significantly improved thyroid indices (TSH, T3, T4) in patients with subclinical hypothyroidism over 8 weeks. This was a single study with 50 participants.
This finding is interesting but preliminary. More importantly, people with thyroid conditions should not self-treat with ashwagandha without medical supervision. Ashwagandha may interact with thyroid medications, and inappropriate supplementation could worsen thyroid disorders. If you have a thyroid condition, discuss ashwagandha with your endocrinologist before trying it.
Claim 6: Cognitive Function
Verdict: Limited evidence. A few positive small trials.
A 2017 study found that KSM-66 at 600 mg/day improved memory and cognitive function in adults with mild cognitive impairment. A 2014 study found improvements in reaction time and task performance in healthy adults. These are small studies, and the cognitive enhancement space is crowded with compounds that have stronger evidence (like creatine for cognitive function under stress).
Safety and Side Effects
Ashwagandha has a good safety profile in clinical trials lasting up to 12 weeks. Common side effects include mild GI discomfort, drowsiness, and headache. However, several important cautions:
- Liver concerns: Rare cases of liver injury have been reported in post-market surveillance. The NIH LiverTox database lists several case reports. While rare, this warrants attention, especially with long-term use
- Thyroid interaction: May affect thyroid hormone levels. Do not combine with thyroid medications without medical guidance
- Pregnancy: Traditional Ayurvedic texts classify ashwagandha as potentially abortifacient. Avoid during pregnancy
- Autoimmune conditions: As an immune modulator, ashwagandha may worsen autoimmune conditions like lupus, rheumatoid arthritis, or multiple sclerosis
- Heavy metals: Ashwagandha products have been flagged in multiple contamination reports for lead and other heavy metals. Third-party testing is essential in this category
Our Assessment
Ashwagandha is a legitimately useful supplement for stress and anxiety reduction, with the evidence to back it up. The sleep benefits are real but work through the stress-reduction mechanism. The testosterone and performance claims are modest and most relevant for stressed or overweight individuals. The thyroid and cognitive claims need more research.
If you decide to take it, use a standardized extract (KSM-66 or Sensoril, not generic powder), buy a product that is third-party tested for heavy metals, and stay within studied dose ranges. See our ashwagandha scorecard for detailed product comparisons scored on evidence, quality, value, and transparency.
For a deeper understanding of how we evaluate supplements, visit our scoring methodology page.