Days' Supply

Days' supply is the number of days a prescription is meant to last, based on the quantity dispensed and the directions. Sixty tablets taken twice a day is a 30-day supply. It is printed on the pharmacy label and sent on every claim, and it is the number the plan uses to decide when you may refill, how much copay you owe, and whether the quantity is within the plan's limit.

It matters more than it looks. Copays are set per fill, so a 30-day supply at $10 costs $120 a year while a 90-day supply at $25 costs $100 a year for the same pills, with a third of the pharmacy trips. Refills are unlocked at a percentage of the days' supply, often 75 to 80 percent, so a 30-day fill can be refilled around day 23. Days' supply is also how adherence is measured: if a member fills a 30-day supply of a blood pressure drug every 45 days, the plan can see the gaps. On the plan's side, quantity limits are written in days' supply too, which is why a prescription for 60 migraine tablets might be cut to 9 a month by the plan's rule.

The takeaway: for any drug you take every day, ask your doctor to write it for 90 days and check what your plan charges for a 90-day fill. It is usually the cheapest legal way to buy the same medicine.