Researchers tested whether ketanserin, a drug that blocks serotonin receptors, could stop or reverse an LSD experience once it had already started. In a controlled study with healthy volunteers, ketanserin given one hour after LSD administration cut the duration of the LSD experience from 8.5 hours down to 3.5 hours and eliminated visual and auditory hallucinations. This finding suggests ketanserin could be useful as an emergency medication in psychedelic-assisted therapy to help patients who have negative reactions to LSD.