In medicine, adherence describes the degree to which a patient correctly follows medical advice. Poor adherence is often associated with patients not following prescribed medicines and treatment regimes. Non-adherence or poor adherence has been reported as a major problem in medical practice and can also affect the outcomes of a clinical trial. Poor adherence, or failure to follow treatment instructions, may be attributed to a number of reasons such as poor communication, forgetfulness, or unpleasant side-effects.

Efforts to improve adherence in clinical practice have included simplifying medication packaging, providing medication reminders, improving patient education, and limiting the number of medications prescribed simultaneously. Various measures of adherence are used in clinical trials, for instance: the assessment of pharmacological response, electronic diaries, residual tablet count, devices to monitor tablet removal from containers, testing for medicine in blood or urine, or the use of pharmacological markers/indicators.