Serotonin syndrome can occur from neurotransmitter affecting drugs such as SSRI's, MAOI's, tricyclics, 5-HPT, etc. When serotonin syndrome actually occurs it is because too much serotonin is released and the reuptake valves are blocked to prevent damage, so too much serotonin is trapped. It usually results in the individual going into a coma. Serotonin syndrome usually doesnt occur from prescribed medications because doctors give strict instructions, however it becomes dangerous when illegal drugs such as MDMA are used, and individulas are even more suseptable when using MDMA in conjunction with other things.