Drug-induced mental disturbances are likely to be encountered in daily clinical practice, but are perhaps not always recognized. This is partly due to the multifactorial causation of psychiatric symptoms. This article deals with the differential diagnosis of unwanted drug effects in psychiatric clinical practice. The drug-related syndromes are classified and examples of drugs with a high incidence are mentioned. The close resemblance of physical side effects, functional complaints and other psychiatric features is discussed. The authors emphasize the importance of a careful consideration of drugs taken by a patient and the need for a systematic evaluation of psychiatric complications of medications. This may be endangered by trends to separate mental en physical public health services.