Introduction:
The placebo/nocebo effect refers to the action of pharmacologically inactive substances, administered in the receptural form of prescriptions (most often pills), which is identical with the form containing therapeutic substances. This effect may be beneficial, neutral, or adverse to a patient. Interdisciplinary studies of this effect indicate the role of expectations as a psychobiological phenomenon; the results of these studies are also important for medical ethics.

Objective:
To draw conclusions concerning the placebo effect which are important for medical practice.

Description of the state of knowledge:
The results of empirical studies confirm that mental processes may activate mechanisms similar to those induced by medicines.

Conclusions:
1.There is a possible conflict between the need for providing a patient with true and full information (autonomy principle), and principles of nonmaleficence and beneficence. 2. Application of the methods of resolving conflicts of values and norms, recommended in professional ethics, is hampered because the term and criteria of a patient›s wellbeing are unclear and blurred; and the definition of health adopted by the WHO is too broad from the practical aspect. 3. Results of clinical tests of the nocebo effect, and also the results of psychological research on the attribution, confirm that the truth and truthfulness norms are not absolute, and in some situations it is justified not to tell a patient the whole truth, which is considered in the postulates regarding the procedures of informing patients (permissibility of not informing). 4. Good communication with a patient may decrease negative effects of medical procedures and the information provided – especially extremely unfavourable diagnoses and prognoses. 5. The consideration of the nocebo effect of medical prognosis speaks for seeking recommendations concerning informing patients of their prognoses (strict procedures are impossible), which would be both professionally and ethically accepted.