Same behavior, different philosophy
Medication adherence and compliance describe the same thing, whether a patient takes a medication as prescribed, but they carry different assumptions. Compliance is the older term. It frames the patient as passively following the clinician's instructions, and the field moved away from it because it implies a one-directional, authority-driven relationship and locates blame on the patient who is labeled non-compliant. Adherence is the preferred modern term. The World Health Organization's 2003 report defined it as the extent to which a person's behavior corresponds with agreed recommendations from a provider. The key word is agreed: adherence presumes the patient is an active partner in the plan, not a subordinate following orders.
Where concordance fits
A third term, concordance, is often confused with the other two but means something different. Concordance, largely a British concept, does not describe the patient's behavior at all; it describes the consultation, the shared decision-making process in which prescriber and patient reach agreement as equals. Adherence is the outcome, whether the patient takes the medication. Concordance is the process, whether the decision to prescribe it was genuinely mutual.
| Compliance | Adherence | Concordance | |
|---|---|---|---|
| What it describes | The patient following orders | The patient following an agreed plan | The prescribing conversation |
| Assumed relationship | Hierarchy: obey | Partnership: agree | Shared decision as equals |
| Status | Older, being retired | Preferred modern term | Chiefly a British concept |
| How it is measured | PDC or MPR | PDC or MPR | Not a measure |
Why the field shifted
The move from compliance to adherence reflected a broader shift in medicine from paternalism toward patient autonomy and shared decision-making, and a recognition that "non-compliance" wrongly placed all the blame on the patient while ignoring system, provider, cost, and condition-related drivers. The WHO's 2003 framing was the inflection point. In everyday practice, payer programs, and the measurement literature, the terms are still used interchangeably, so the distinction matters most for tone and framing.
The measurement is identical
Whatever you call it, the math is the same. Both compliance and adherence are operationalized through claims-based refill metrics. The standard is Proportion of Days Covered (PDC), the share of days in a period that a patient had the medication on hand, with a PDC of 80% or higher counting as adherent. The older Medication Possession Ratio can exceed 100% when patients refill early, so it overstates adherence, which is why PDC became the standard. A vendor's "compliance program" and a plan's "adherence program" almost always rest on the same number: PDC at the 80% line.
Which term Medicare uses
Medicare uses adherence. CMS adopted the Pharmacy Quality Alliance's PDC measures as the Part D Star Ratings measures, with the diabetes, hypertension, and cholesterol measures each triple-weighted and an 80% PDC threshold defining an adherent member. For anyone working in value-based care, adherence is the correct term of art: it is the language used by the WHO, CMS, and modern guidelines, it aligns with patient-engagement framing, and it is the literal name of the revenue-relevant Star measures.
Why it matters operationally
The terminology debate is real but mostly semantic. The practical takeaway is to standardize on adherence and focus on the measurement, because that is what moves a plan's rating. The work is the same regardless of the label: detect a refill gap early, reach the patient or prescriber, resolve the barrier, and confirm the fill before the gap drops the member below 80% for the year. See medication adherence for the full measure and how to improve Part D adherence for the operating model.