The disturbances of serotonergic function may be a factor in hallucinogen-persisting perception disorder, although the pathophysiology remains unclear. Regarding the treatment of LSD-induced flashbacks, the choice of medication is still controversial. The use of various agents, including neuroleptics, serotonin reuptake inhibitors (SSRIs), anticonvulsants, and benzodiazepines, has met with limited success (2). Some researchers report that risperidone, which is a serotonin-dopamine antagonist, exacerbates symptoms of hallucinogen-persisting perception disorder (3). Others note that SSRIs exacerbate flashbacks (4). In this case, his perceptual disturbance symptoms responded to risperidone treatment.