Review of clinical studies on the nocebo effect.

: To review clinical studies on the nocebo effect. PubMed was searched for relevant clinical studies as well as studies on the relationship between the nocebo effect and genes. : A total of 35 clinical studies on the nocebo effect and one study on its relationship with genes were selected for review. All were conducted outside Japan. : An increasing number of clinical studies on the nocebo effect are being published. The 36 studies selected for review were grouped into the following five categories: (1) studies of how differences in participant characteristics such as personality affect susceptibility to the nocebo effect, (2) studies of how differences in provision of information about side effects affect susceptibility to the nocebo effect, (3) studies of how nocebo conditioning affects susceptibility to the nocebo effect, (4) studies of nocebo response mechanisms, and (5) studies of the nocebo effect and genetic polymorphisms. The first four categories comprised 5, 19, 8, and 3 studies, respectively, and the fifth comprised 1 study. Most of the studies investigated how differences in the provision of information affect susceptibility to the nocebo effect. Few studies investigated individual differences in the nocebo effect (differences between responders and non-responders) or mechanisms of the nocebo effect.