The patient perspective in the process of drug evaluation and drug use is high on the agenda, which is demonstrated by an increased use of patient-reported outcome instruments to evaluate drugs and a shift towards patient-centred care in clinical practice. This thesis contains studies focusing on 1) the development and validation of a patient-reported outcome instrument to assess adverse drug events (ADEs), and 2) the role of patient characteristics and preferences on treatment decisions in clinical practice. The first part presents the development of a generic questionnaire to assess ADEs from the patient perspective. Although this questionnaire showed sufficient content and concurrent validity to detect ADEs at a general level, it was not sensitive enough to detect all ADEs perceived by patients. Suggestions are provided to improve the questionnaire for future use. In the second part, insight in decisions to start or intensify treatment with special attention for different patient age groups is provided. It was found that age influenced prescribing behaviour as well as the patient's willingness to add a drug. For all patients, preventing death and ADEs were important considerations when choosing an additional drug. The influence of beliefs about benefits and risks on patients’ drug adherence, however, differed among types of drugs. These findings can be used to improve the assessment of ADEs from the patient perspective, to incorporate the patient perspective in treatment decisions and to develop better tailored interventions for improving drug adherence.