Aims and methodTo define serotonin syndrome and its symptoms and to discover which drugs or drug combinations are likely to cause it. A review of literature (including case reports) relating to serotonin syndrome collated from searches of MedLine and Micromedex covering the period January 1991 to July 1998.ResultsMost of the data found were either individual case reports or reviews of case reports. Reports of serotonin syndrome seem to be growing, certainly since the introduction of selective serotonin reuptake inhibitors. Particular combinations seem most likely to induce serotonin syndrome. Awareness of this syndrome as a distinct clinical entity seems to be growing.Clinical implicationsSerotonin syndrome is more likely to occur with drug combinations, especially those involving monoamine oxidase inhibitors. It can also occur when swapping antidepressant therapy, especially If changing from a long acting antidepressant such as fluoxetine. Caution is needed when changing antidepressants and particularly when they are used in combination.