Serotonin syndrome

Serotonin syndrome, more precisely called serotonin toxicity, is a drug-induced condition caused by excessive serotonin activity in the nervous system. It classically presents as a triad: neuromuscular abnormalities (tremor, muscle rigidity, overactive reflexes, and clonus — rhythmic involuntary muscle contractions, the most telling sign), autonomic hyperactivity (fever, sweating, rapid heart rate, high blood pressure), and altered mental state (agitation, confusion, restlessness). It exists on a spectrum from mild and easily missed to a rapidly progressing, fatal emergency. There is no lab test that confirms it — diagnosis is clinical. Onset is usually fast, often within hours of the triggering dose or combination.

Serotonin syndrome is the specific danger behind many 'do not combine' warnings in harm reduction. It most often occurs when two or more serotonin-raising substances are taken together, especially if they raise serotonin by different mechanisms — the highest-risk combination is a monoamine oxidase inhibitor (MAOI) with an SSRI, SNRI, or another serotonergic drug. Substances that commonly contribute include MDMA and MDA, other stimulants, tramadol, meperidine (pethidine), dextromethorphan (DXM), St. John's Wort, triptans, and antidepressants (SSRIs, SNRIs, MAOIs, tricyclics). A critical and under-recognized case is ayahuasca, which contains MAOIs — combining it with other serotonergic drugs (SSRIs, MDMA, other stimulants) can be acutely dangerous. It can also occur from a single serotonergic drug taken in overdose. Because some drugs (like fluoxetine, and irreversible MAOIs) stay active for weeks, a dangerous interaction can occur even weeks after stopping one drug. Warning signs to act on: agitation or confusion, heavy sweating and fever, shivering or muscle twitching/jerking, a racing heart, and muscle rigidity. Severe cases cause dangerously high body temperature, seizures, and organ failure. This is a medical emergency — if someone shows these signs after taking serotonergic drugs, call 911 immediately; Poison Control (1-800-222-1222) can advise. Do not wait for the full triad to appear. This page has not yet been medically reviewed.