Medication adherence is  the extent to which a patient follows the orders of a healthcare professional in consuming their medication. Generally speaking, adherence may refer to any other physician-prescribed regimen, including exercise, lifestyle recommendations, therapy, and self-care.

Nonadherence (sometimes called noncompliance) can take several forms, including medical advice being misunderstood, ignored, carried out incorrectly, or forgotten. Nonadherence has large repercussions on both physical and economic well being. Each year, the cost of nonadherence reaches hundreds of billions of US dollars, as well as up to 125,000 deaths.