Abstract The resurgence of interest in using psychedelic drugs, including lysergic acid diethylamide (LSD), in psychiatry has drawn attention to the medically unsupervised practice of ‘microdosing’. Thousands of users claim that very low doses of LSD, taken at 3–4‐day intervals, improve mood and cognitive function., However, few controlled studies have described the effects of the drug when taken in this way. Here, in a double‐blind controlled study, we studied the effects of four repeated doses of LSD tartrate (13 or 26 μg) or placebo, administered to healthy adults at 3–4 day intervals, on mood, cognitive performance and responses to emotional tasks. Participants were randomly assigned to one of three drug conditions: placebo (N = 18), 13 μg LSD (N = 19), or 26 μg LSD (N = 19). They attended four 5‐hour drug‐administration sessions separated by 3–4 days, followed by a drug‐free follow‐up session 3–4 days after the last session. LSD (26 μg) produced modest subjective effects including increased ratings of ‘feeling a drug effect’ and both stimulant‐like and LSD‐like effects, but the drug did not improve mood or affect performance on psychomotor or most emotional tasks. No residual effects were detected on mood or task performance on the drug‐free follow‐up session. We conclude that within the context of a controlled setting and a limited number of administrations, repeated low doses of LSD are safe, but produce negligible changes in mood or cognition in healthy volunteers.

Repeated low doses of LSD in healthy adults: A placebo‐controlled, dose–response study