The phenomenon where a patient experiences adverse effects from an inert treatment (or attributes unrelated symptoms to a treatment) because they expect negative outcomes. The nocebo effect is the mirror image of the placebo effect: belief in harm produces harm, just as belief in benefit produces benefit.

Nocebo effects are measurable and significant. In clinical trials, placebo groups regularly report side effects—sometimes at rates approaching the active treatment group. This complicates safety assessment: when 15% of the treatment group reports nausea and 12% of the placebo group does too, the actual drug-attributable nausea rate is closer to 3%. Nocebo effects are particularly relevant when evaluating community-reported side effects from peptides, where expectation bias is high and controlled comparisons are absent.