Serotonin syndrome (SS) consists of a group of symptoms that may occur with the use of certain serotonergic medications or drugs. The symptoms can range from mild to severe, and are potentially fatal. Symptoms in mild cases include high blood pressure and a fast heart rate, usually without a fever. Symptoms in moderate cases include high body temperature, agitation, increased reflexes, tremor, sweating, dilated pupils, and diarrhea. In severe cases, body temperature can increase to greater than 41.1 °C (106.0 °F). Complications may include seizures and extensive muscle breakdown.
Serotonin syndrome is typically caused by the use of two or more serotonergic medications or drugs. These may include selective serotonin reuptake inhibitors (SSRIs), serotonin norepinephrine reuptake inhibitors (SNRIs), monoamine oxidase inhibitors (MAOIs), tricyclic antidepressants (TCAs), amphetamines, pethidine (meperidine), tramadol, dextromethorphan, buspirone, L-tryptophan, 5-hydroxytryptophan, St. John's wort, triptans, MDMA, metoclopramide, or cocaine. It occurs in about 15% of SSRI overdoses. It is a predictable consequence of excess serotonin on the central nervous system. Onset of symptoms is typically within a day of the extra serotonin.