Multiple small to medium-sized RCTs show that ashwagandha can reduce stress and anxiety and lowers cortisol, but one study found no effect on the primary stress measure. Short-term safety appears good, but long-term effects are unknown.
Multiple double-blind, placebo-controlled studies show that ashwagandha extract can reduce perceived stress and anxiety. In an RCT with 60 participants (240 mg/day, 60 days), anxiety scores measured via the Hamilton Anxiety Rating Scale fell significantly compared to placebo, and morning cortisol (the primary stress hormone in the blood) also dropped sharply. This points to an effect on the so-called HPA axis, the body's biological stress-regulation axis (PMID 31517876).
Another RCT (500 mg/day, 60 days, n=50 completers) found similar results: stress, anxiety and quality of life all improved significantly, and cortisol decreased while urinary serotonin increased. A note of caution: the researchers in this study were employed by the manufacturer of the extract used, which represents a clear conflict of interest. The results should therefore be interpreted more cautiously (PMID 37832082).
Not all studies are positive. In a 12-week RCT in obese, older adults (40-75 years), ashwagandha (400 mg/day) did not improve the stress score significantly better than placebo. There was, however, less fatigue and better heart rate variability. This is an important counterweight: the supplement does not always work and does not work for everyone (PMID 37740662).
An Indian dose-finding RCT (n=98) showed that as little as 125 mg per day significantly reduces stress, and that higher doses work somewhat better. A recent RCT (n=90, 84 days, 125 mg/day) confirmed improvements in stress, anxiety and sleep quality, but found no significant change in blood cortisol levels. That is noteworthy: subjective complaints improved while the objective hormonal marker did not, which leaves the precise mechanism of action unclear (PMID 38732539, PMID 40875185).
A systematic review of 9 clinical studies concludes that short-term ashwagandha supplementation consistently lowers cortisol in stressed individuals. At the same time, a warning is raised that no one has investigated what prolonged cortisol suppression does to adrenal function over the long term (PMID 38140274). A safety review of 30 human studies found no serious adverse effects. The most commonly reported mild complaints are drowsiness, stomach discomfort and loose stools. Long-term safety (beyond 8 months) has not been well studied (PMID 32201301).
Finally: ashwagandha can also influence thyroid hormones and sex hormones. This may be beneficial for some people, but it is also a risk signal. People with thyroid conditions, hormonal disorders, or who are taking hormone-influencing medication should consult a doctor before starting (PMID 38003702).
9 claims based on 8 unique PMIDs: 6 RCTs (1 of which has a manufacturer conflict of interest), 1 systematic review of 9 studies, 1 safety review of 30 studies, 1 narrative review on hormones. No large, independent meta-analysis is available as a source. Study sizes are small to medium (n=50-131). Short-term studies (8-12 weeks); long-term data are lacking.