A nocebo effect is said to occur when a patient's expectations for a treatment cause the treatment to have a worse effect than it otherwise would have. For example, when a patient anticipates a side effect of a medication, they can experience that effect even if the "medication" is actually an inert substance. The complementary concept, the placebo effect, is said to occur when expectations improve an outcome.
More generally, the nocebo effect is falling ill simply by consciously or subconsciously anticipating a harmful event. This definition includes anticipated events other than medical treatment. It has been applied to Havana syndrome, where purported victims were anticipating attacks by foreign adversaries. This definition also applies to cases of electromagnetic hypersensitivity.
