How it is recognized

A 1960 report described neurologic effects in patients given the amino acid tryptophan while taking an MAO inhibitor. In 1991 Harvey Sternbach proposed diagnostic criteria based on 38 reported cases. The Hunter criteria, developed in 2003 from 2,222 admissions for overdoses of serotonergic drugs, proved more sensitive than Sternbach’s (84 vs 75 percent) and slightly more specific (97 vs 96 percent). They rely mainly on clonus (involuntary, rhythmic muscle jerking), together with signs such as agitation, sweating, tremor, overactive reflexes or fever.

There is no laboratory test, and because the criteria were built for overdoses and drug interactions, applying them loosely to people on stable medication can mislabel ordinary side effects as toxicity. Severe toxicity is a medical emergency.

Psilocybin and antidepressants: what studies found

Psilocybin acts mainly on serotonin 5-HT2A receptors. A 2022 review concluded that true serotonin toxicity usually involves an overdose or an MAO inhibitor, and that antidepressants without MAO inhibition carry a low risk when combined with psychedelics that also contain none. Ayahuasca is different: it contains MAO-inhibiting plant compounds, and taken with an SSRI it may cause serotonin syndrome.

In a placebo-controlled crossover study, 23 healthy volunteers took psilocybin twice: once after two weeks of the SSRI escitalopram and once after placebo. Positive effects were similar, while anxiety, unpleasant drug effects and cardiovascular effects were lower after escitalopram. Among 19 people with treatment-resistant depression who kept taking an SSRI, a single psilocybin session led to no serious adverse events over three weeks of follow-up. More often the reported interaction is a weaker effect, not a dangerous one: in an online survey, people taking SSRIs described mushroom effects as weaker than expected in nearly half of reports. These studies are small and cannot rule out rare reactions.

How it differs from other risks

Lithium carries a separate risk: in online reports of lithium taken with a classic psychedelic, the problem reported most often was seizures, which is a different hazard from serotonin toxicity. Stopping an antidepressant is not free of risk either, and in one exploratory analysis people who had recently stopped responded less well to psilocybin therapy. None of these findings is guidance on combining or stopping a medicine; that decision belongs with the prescriber. Our safety and screening page explains how we handle medication reviews.

Related terms

Psilocybin Contraindications: conditions and medicines that rule psilocybin out.

Set and Setting: what shapes the experience besides the drug and its dose.