Placebo effects. Issues for clinical practice in psychiatry and medicine.

Placebo effects are integral to everyday clinical practice; therefore, they should be well understood by all health care practitioners. Despite the rich literature on the topic, placebo effects receive only passing mention in major textbooks of psychiatry and medicine. The authors clarify the placebo construct and offer a selective review of its history, definitions, mechanisms, and relation to experimental methodology and statistics. Also considered are the concept of nocebo, variation in placebo response rates, and some economic and ethical problems with placebos in clinical trials. Directions are suggested for future research.