Non compliance is a frequent and underestimated problem in clinical practice, that is associated with considerable risks, adverse reactions and costs. Next to omitting one or several doses with consequent lack of efficacy, other forms of non compliance can be described. These include administration of a wrong dose or at a wrong time, early termination of therapy or also its unwarranted continuation, and self-administered comedication without consideration of potential interactions. A number of risks and adverse effects can be derived from these different forms of non compliance, for which we present several examples from clinical practice and the literature. Anticipation, recognition and appropriate countermeasures are important elements for the prevention of non compliance, which may target the pharmacotherapy itself, the patient, or the health system providing the therapeutic framework. Furthermore, a climate of trust and good communication between the patient and the health care provider is the cornerstone for all strategies that aim to improve compliance. Computerized physician order entry and clinical decision support systems may have an additional important role for the prevention of non compliance in the future.