Significance Statement
Lysergic acid diethylamide (LSD) is being investigated in psychedelic-assisted therapy. Therapy sessions with LSD are long due to its duration of acute action of 8–11 hours. Ketanserin prevents effects of LSD when given before LSD. However, unknown is whether LSD’s effects can also be blocked once established. Therefore, we tested whether ketanserin (40 mg) does reduce the duration of action of LSD when given 1 hour after LSD (100 μg). In line with our hypothesis, ketanserin effectively reduced the duration of action of LSD from 8.5 to 3.5 hours. We conclude that ketanserin can be useful in LSD-assisted therapy to shorten the acute response to LSD as a planned or emergency treatment and therefore make this form of therapy safer and/or more flexible.