What is serotonin syndrome and what causes it?

Serotonin syndrome is a potentially life-threatening condition caused by excessive serotonin activity in the nervous system. It presents with a triad of mental status changes like agitation and confusion, autonomic instability including fever, tachycardia, and diaphoresis, and neuromuscular hyperactivity such as tremor, clonus, and hyperreflexia. It most commonly occurs when two or more serotonergic medications are combined, such as an SSRI with a monoamine oxidase inhibitor, tramadol, or the herbal supplement St. John's wort.