What the human trials show about ashwagandha and cortisol: the KSM-66 and Sensoril studies, the doses tested, how long they ran and their limits.
In small placebo-controlled trials, ashwagandha extracts lowered cortisol and stress scores in stressed adults. In a 60-day trial of 64 adults (Chandrasekhar 2012), 300 mg of KSM-66 twice daily lowered serum cortisol about 28% from baseline, and an 8-week, maker-funded trial of 98 stressed adults found Sensoril at 125 to 500 mg a night lowered cortisol more than placebo. The trials were small and mostly funded or supplied by the extract makers, so treat the results as promising rather than settled.
The hypothalamic-pituitary-adrenal (HPA) axis is the body's central stress response system. When the brain perceives stress, the hypothalamus releases CRH → the pituitary releases ACTH → the adrenal glands secrete cortisol. Long-term stress can keep cortisol raised, which is associated with anxiety, poor sleep, weight gain and, in some studies, lower testosterone.
Ashwagandha's active compounds, called withanolides, are thought to act on the HPA axis, but the exact mechanism in people isn't established. What the trials measured is the outcome: cortisol and stress scores fell more on ashwagandha than on placebo. Different extracts were tested at different doses. KSM-66 (root only) was usually given at 300 mg once or twice a day, and Sensoril (root and leaf) at 125 to 500 mg a day, so match the dose to the extract you buy.
Chronic stress and high cortisol are linked with lower testosterone, but the popular idea that cortisol "steals" shared precursors such as pregnenolone is an oversimplification. KSM-66 raised testosterone modestly in a small 8-week trial of young men doing resistance training (600 mg a day), but the 2012 cortisol trial did not measure testosterone, so lower cortisol shouldn't be assumed to mean higher testosterone. Ashwagandha isn't for everyone: it has been linked to rare liver injury, can raise thyroid hormone levels and may interact with sedative, diabetes, blood pressure and thyroid medicines, and it should be avoided in pregnancy.