The nocebo effect is the phenonemon that a patient who believes a treatment will cause harm, will actually experience adverse effects.

The nocebo effect is distinguished from older concepts such as sociogenic illness and psychosomatic illness, which are broader terms where a known or unknown cause with cultural or psychological factors is involved. Distinguishing between these effects can be a difficult problem for medical semiotics, or the study of how to interpret the patient's description of their ailments.

The nocebo effect is the reverse of the well-known placebo effect.