In medicine, patient compliance (also adherence, capacitance) describes the degree to which a patient correctly follows medical advice. Most commonly, it refers to medication or drug compliance, but it can also apply to other situations such as medical device use, self care, self-directed exercises, or therapy sessions. Both patient and health-care provider affect compliance, and a positive physician-patient relationship is the most important factor in improving compliance. Access to care plays a role in patient adherence, whereby greater wait times to access care contributing to greater absenteeism. The cost of prescription medication also plays a major role.

Compliance can be confused with concordance, which is the process by which a patient and clinician make decisions together about treatment.

Worldwide, non-compliance is a major obstacle to the effective delivery of health care. 2003 estimates from the World Health Organization indicated that only about 50% of patients with chronic diseases living in developed countries follow treatment recommendations with particularly low rates of adherence to therapies for asthma, diabetes, and hypertension. Major barriers to compliance are thought to include the complexity of modern medication regimens, poor "health literacy" and not understanding treatment benefits, the occurrence of undiscussed side effects, poor treatment satisfaction, cost of prescription medicine, and poor communication or lack of trust between a patient and his or her health-care provider. Efforts to improve compliance have been aimed at simplifying medication packaging, providing effective medication reminders, improving patient education, and limiting the number of medications prescribed simultaneously. Studies show a great variation in terms of characteristics and effects of interventions to improve medicine adherence. It is still unclear how adherence can consistently be improved in order to promote clinically important effects.