This paper describes the personal views of the author about diagnosis and management of an adverse drug effect. It proposes that diagnosis is complicated and is also supported by carefully observed management of changes in drug therapy. Drug-related adverse effects may be due to the drug itself, though many are due to systematic errors occurring in the process from diagnosis of the primary treated condition, through prescribing and dispensing, to the way the drug is used by the patient. Bringing awareness of such systematic errors for consideration and management is part of a health care professional's responsibilities.