Ashwagandha has more clinical trial data behind it than most herbal supplements, but the evidence is stronger for some benefits than others. It reliably lowers cortisol, the body’s primary stress hormone, by a meaningful amount. It improves objective sleep measures. It shows promise for memory, physical endurance, and thyroid function. But some of its most popular claimed benefits, like reducing the feeling of being stressed, haven’t held up as well under rigorous analysis.
Here’s what the research actually shows, benefit by benefit.
Cortisol Drops, but Perceived Stress May Not
The strongest evidence for ashwagandha involves cortisol. A systematic review and meta-analysis of controlled trials found that ashwagandha supplementation reduced blood cortisol levels by an average of 1.16 µg/dL, a statistically significant result. That’s a real, measurable change in the hormone most directly tied to your body’s stress response.
Here’s the catch: the same meta-analysis found no significant impact on perceived stress, meaning participants didn’t consistently report feeling less stressed on standardized questionnaires. This is a genuinely important distinction. Your body’s stress chemistry changes, but you may not notice a dramatic shift in how stressed you feel day to day. It’s possible the cortisol reduction contributes to downstream benefits like better sleep and recovery without producing an obvious “I feel calmer” sensation. It’s also possible that longer supplementation periods or higher doses are needed for subjective effects to emerge, but the current pooled data doesn’t support the claim that ashwagandha reliably makes you feel less stressed.
Sleep Improvements Are Well Supported
Sleep is where ashwagandha’s evidence looks particularly solid. A randomized, double-blind, placebo-controlled trial of 144 healthy adults found a 72% increase in self-reported sleep quality in the ashwagandha group, compared to 29% in the placebo group. That gap is large and statistically significant.
More importantly, this wasn’t just people saying they slept better. The study used activity monitors to track sleep objectively. After six weeks, the ashwagandha group showed significant improvements in sleep efficiency (the percentage of time in bed actually spent sleeping), total sleep time, how quickly they fell asleep, and how often they woke up during the night. All four measures improved compared to placebo. These are the kinds of objective outcomes that are hard to attribute to expectation alone.
Memory and Attention Show Early Promise
A randomized, double-blind trial tested ashwagandha’s effects on cognitive function in stressed but otherwise healthy adults using a validated computerized test battery. The ashwagandha group recalled significantly more patterns correctly on their first attempt (scoring 12.9 versus 10.1 in the placebo group) and made fewer total errors (17.5 versus 27.7). They also performed better on a measure of sustained attention.
A separate trial in 50 adults with mild cognitive impairment found significant improvements in immediate memory, general memory, executive function, attention, and processing speed over eight weeks. These results are encouraging, though the number of participants in individual studies remains relatively small. The cognitive evidence is real but still building.
Physical Performance Gets a Modest Boost
A systematic review and meta-analysis of aerobic fitness trials found that ashwagandha significantly enhanced VO2 max, a measure of how efficiently your body uses oxygen during exercise. The average improvement was about 3 mL/kg/min. For context, that’s a modest but meaningful gain, roughly equivalent to what you might see from a few weeks of consistent cardio training. The effect was more pronounced in athletes than in sedentary adults.
The mechanism likely involves ashwagandha’s anti-fatigue properties. Several studies noted improvements in time to exhaustion, suggesting people could exercise longer before giving out. Study durations ranged from 2 to 12 weeks, with doses between 300 and 1,000 mg per day. One caveat: heterogeneity across studies was high, meaning results varied quite a bit depending on the specific trial design and population.
Thyroid Hormones Respond Significantly
Ashwagandha stimulates thyroid hormone production, and this is both a benefit and a risk depending on your situation. In a double-blind, placebo-controlled trial of people with mildly underactive thyroids, eight weeks of ashwagandha significantly improved levels of both thyroid hormones (T3 and T4) while bringing TSH, the pituitary signal that tells the thyroid to work harder, back toward normal. All three measures changed significantly compared to placebo.
This thyroid-stimulating effect means ashwagandha could be genuinely helpful for subclinical hypothyroidism. But it also means people with overactive thyroids, or those taking thyroid medication, need to be cautious. At least one case report documented a young woman developing symptoms of thyroid excess, including weight loss, shakiness, and heart palpitations, after starting ashwagandha. Her thyroid levels normalized after stopping.
How It Works in the Body
Ashwagandha’s primary mechanism involves the communication loop between your brain and adrenal glands, often called the HPA axis. This is the system that ramps up cortisol production when you’re under stress. Ashwagandha appears to dampen this axis, reducing cortisol output. The active compounds, called withanolides, are the plant chemicals most directly responsible for these effects.
Beyond cortisol, ashwagandha influences reproductive hormones. In men, it increases luteinizing hormone and follicle-stimulating hormone, which are involved in testosterone production and fertility. It also acts on the thyroid axis, increasing T3 and T4 production. These aren’t isolated effects. They’re part of the same interconnected hormonal network, which explains why ashwagandha’s influence shows up across so many seemingly unrelated outcomes.
Dosing Used in Clinical Trials
There’s no single agreed-upon dose, but the range used in successful trials is fairly consistent. Most studies used root extract at 300 to 600 mg per day, standardized to contain at least 5% withanolides. A joint international taskforce from the World Federation of Societies of Biological Psychiatry and the Canadian Network for Mood and Anxiety Treatments provisionally recommends 300 to 600 mg per day of root extract (standardized to 5% withanolides) for anxiety.
Some studies have gone lower (120 mg per day of a concentrated root and leaf extract) or higher (1,250 mg per day), but the 300 to 600 mg range of root extract is the sweet spot where most positive results cluster. The United States Pharmacopeia defines a standardized ashwagandha root extract as containing no less than 2.5% withanolides, so look for products that meet or exceed this threshold. Study durations typically ranged from 4 to 12 weeks before benefits became measurable.
Choosing Between Extract Types
Two branded extracts dominate the market, and they’re meaningfully different. KSM-66 is made exclusively from roots, standardized to 5% withanolides. It keeps levels of withaferin A, a potentially harmful compound, very low. This is the extract used in many of the sleep, cognition, and fitness studies. Sensoril uses both roots and leaves to achieve a higher total withanolide concentration (at least 10%), but including leaves increases withaferin A content. Sensoril’s research has focused more on stress reduction and cortisol.
The root-only approach aligns with traditional use and with the pharmacopeial standards from India, Britain, and the United States, all of which specify the root. This doesn’t make leaf-containing extracts ineffective, but it does mean the safety profile is better established for root extracts.
Safety and Known Risks
In clinical trials, ashwagandha has a clean safety record. Side effects are uncommon, and no trials have reported significant liver enzyme elevations or serious adverse events. Large doses can cause digestive discomfort, nausea, or diarrhea from direct irritation of the gut lining.
Outside of trials, however, the picture is slightly more complicated. Since 2017, an increasing number of case reports have linked commercial ashwagandha products to liver injury. These cases typically appeared 2 to 12 weeks after starting the supplement, with symptoms including jaundice and itching. Most cases resolved after stopping, though rare fatal outcomes have been reported, particularly in people who already had liver disease. Because commercial supplements sometimes contain undisclosed ingredients or contaminants, it’s not always clear whether ashwagandha itself caused the damage. In several cases, though, testing confirmed the product contained only ashwagandha. The NIH’s LiverTox database rates ashwagandha as a “likely” cause of clinically apparent liver injury, though the absolute risk remains low given its widespread use.
People with existing liver disease, particularly cirrhosis, should avoid ashwagandha. Those with thyroid conditions should be aware of its hormone-stimulating effects, which could interfere with medication or worsen hyperthyroidism.