Lysergic acid diethylamide (LSD) acts as an antagonist at peripheral 5-HT receptor subtypes, but as a 5-HT 2A receptor agonist in the CNS. In severe poisoning, hyperreflexia, tremor, muscle twitching, coma, seizures, hypotension and respiratory arrest have been reported. In addition, changes in perception, mood and behaviour, panic, agitation and excitement, visual hallucinations, delusions and psychosis are observed. Hallucinogen persisting perception disorder (‘flashbacks’) may persist for several years after exposure has ceased.