Negative previous experiences (conditioning), observational learning and expectations can trigger nocebo effects. They are responsible for a substantial proportion of the side effects of medical treatment, and are induced primarily by risk information for informed consent. This has been demonstrated in studies on patient groups with or without modified medical information. Drug trials in the field of neurology and psychiatry have regularly revealed side effects and drop-out also in placebo groups. The underlying neurobiological mechanisms were experimentally investigated especially for nocebo-induced hyperalgesia, whereby the specifically involved regions of the brain could be identified by functional imaging as well as changes in the dopamine, endorphin and cholecystokinin systems. Nocebo effects are specific, i.e. they are closely related to patient information and can induce or aggravate exactly the symptoms addressed. Nevertheless, informed consent is an essential part of doctor-patient communication; however, information on risks can be markedly less damaging when unnecessary repetitions are avoided, misunderstandings are recognized or resolved and several different options are given. In addition, risks should always be named together with positive aspects, such as the advantages of the appropriate treatment, the prophylactic measures applied or the early detection and treatment of developing side effects. The best protection against harm caused by information on risks is a trustful doctor-patient relationship. Poor knowledge of nocebo effects or lack of countermeasures constitute a serious threat to patients and according to the current state of knowledge could be rated as medical malpractice.