How Pharmacies Calculate "Days Supply" and Why It Matters
When an insurance provider or pharmacist processes your medication, they compute a fundamental metric known as Days Supply. The formula is straightforward: Days Supply = Total Units Dispensed / Daily Prescribed Dose.
For example, 60 tablets taken twice daily (BID) yields a 30-day supply. Health insurance plans generally prohibit refilling a prescription until 75% to 85% of the days supply has elapsed (commonly around day 23 to 25 of a 30-day fill). Submitting your refill request within this 5-day window prevents both pharmacy backorders and insurance denial.
The 80% PDC Standard: The Clinical Line Between Efficacy and Complications
In medical literature, PDC (Proportion of Days Covered) measures the percentage of days a patient actually has medication in hand during a treatment period. Over 500 clinical studies have validated that 80% is the critical clinical threshold:
| Medication Class | Adherence Rate ≥ 80% | Adherence Rate < 80% | Clinical Consequence |
|---|---|---|---|
| Blood Pressure (Antihypertensives) | Stable vascular pressure | Rebound hypertension | 3.8× higher risk of cardiovascular events. |
| Diabetes (Oral Hypoglycemics) | HbA1c target < 7.0% | Hyperglycemic spikes | Accelerated microvascular neuropathy & kidney stress. |
| Statins (Cholesterol) | Plaque stabilization | Elevated LDL-C | Increased risk of myocardial infarction. |
| Antidepressants / SSRIs | Neurotransmitter balance | Discontinuation syndrome | Brain zaps, acute mood crashes, insomnia. |