Across nine trials in 558 people, pooled in one review, cortisol fell against placebo, and so did self-rated stress and anxiety. The people studied were already highly stressed when they started.
The trials recruited adults who already described themselves as highly stressed, so the same fall may not appear in someone whose cortisol starts out ordinary. Four of the nine trials reported mild-to-moderate side effects. Extract and dose differed from trial to trial, so the pooled result describes no one product. The second review's two studies that measured cortisol in saliva rather than blood disagreed, one finding no change at all. In a later 12-week trial of older, overweight adults, stress scores fell about as far on placebo as on ashwagandha, and cortisol was not measured, so the self-rated stress part of this result is less settled than the cortisol part.