It pays to shop for bargains, even at the pharmacy counter. Seniors who signed up for a Medicare Part D drug plan would ordinarily expect to pay a predetermined price set by their plan for prescription drugs. But get this: If the pharmacy's cash price (the amount it charges someone without insurance) is lower than the drug plan's price, seniors can pay the lower rate as long as they're using an in-network pharmacy. That's the message of a recent notice from the Centers for Medicare and Medicaid Services.

"We've seen this happening with some individuals," says Vicki Gottlich, a policy analyst at the Center for Medicare Advocacy, a nonprofit organization that assists Medicare recipients. In one case, she says, a man's Part D plan charged a $30 copayment for the seizure drug Dilantin, while the pharmacy's cash price was $27. In addition, some seniors may have state-sponsored or other types of discount cards that give them a lower rate than their plan charges for drugs.

Seniors on a budget are naturally interested in saving money. But there's another reason to keep an eye on drug costs: avoiding or minimizing spending in the "donut hole." The way Part D is structured, once a senior's drug costs surpass $2,250 in a year, he or she must pay for the drugs until total costs reach $3,600. After that, the program's catastrophic coverage kicks in. A plan member who pays less for prescription drugs will take longer to reach that hole in coverage. At the same time, seniors who do reach the coverage gap and about a quarter of them will do so will pay less out of pocket if the pharmacy's cash price for a drug is lower than their plan's negotiated price.

How much a pharmacy charges for a particular drug on any given day may vary. For example, the druggist may have an arrangement to get a discount on one drug if it buys another from the same manufacturer, says Meredyth Andrus, an assistant attorney general in Maryland's Office of the Attorney General who works on pharmaceutical antitrust cases. Or the drug's manufacturer or distributor may change the cost of a drug. "Drug prices can vary by several dollars from pharmacy to pharmacy and from one day to the next," says Andrus. What that means for seniors: Shopping around and checking prices with pharmacists before pulling out your Medicare drug card may pay off.

Seniors who pay cash for a covered drug to take advantage of a lower price should be sure to save the receipt and contact their plan to find out how to get reimbursed. Each plan may have a slightly different process, but they're required to pay seniors back for the drug as long as they use a network pharmacy.