Compliance (or adherence) is a medical term that means the degree to which a patient correctly follows medical advice. Most commonly, it refers to medication, or drug compliance, but may also mean use of medical appliances such as compression stockings, chronic wound care, self-directed physiotherapy exercises, or attending counseling or other courses of therapy. The most effective way for a doctor to improve patient compliance is through a positive physician-patient relationship.

Other factors that increase compliance include:

• Patient feeling ill
• Limitations of patients activities due to disease state
• Written instructions for taking medication
• Acute illness
• Simple treatment schedule
• Short time spent in waiting room
• Physician recommending one change at a time
• Benefits of care outweigh costs
• Peer support

Patients may not accurately report back to health care workers because fear of possible embarrassment, being chastised, or seeming to be ungrateful for a doctor's care.

Causes for poor compliance include:

• Forgetfulness
• Poor rapport with physician
• Few symptoms
• Chronic illness
• Prescription not collected or not dispensed
• Purpose of treatment not clear
• Perceived lack of effect
• Real or perceived side-effects
• Instructions for administration not clear
• Physical difficulty in complying (e.g. opening medicine containers, handling small tablets, swallowing difficulties, travel to place of treatment)
• Unattractive formulation, such as unpleasant taste
• Complicated regimen
• Cost of drugs

Some prescription drugs may also be used by the patient in doses higher than recomended theraputic dose for recreational effects of sold for profit.