Medicare officials say it's "unacceptable" for insurers to put "gag orders" in pharmacy contracts, keeping pharmacists from alerting seniors about when they could save money by paying cash — rather than using insurance — for their prescriptions.

The agency is warning that such provisions run counter to President Donald Trump's mission to bring down prescription drug costs and promote transparency about drug prices. Yet, it has stopped short of requiring insurers to lift such restrictions on pharmacists.

That doesn't mean people with Medicare drug coverage are destined to overpay for prescriptions. Under a little-known Medicare rule, they can pay a lower cash price for prescriptions instead of using their insurance. But first, they must ask the pharmacist about that option, said Julie Carter, federal policy associate at the Medicare Rights Center, a patient advocacy group.

"If they bring it up, then we can inform them of those prices," said Nick Newman, a pharmacist and the manager at Essentra Pharmacy in rural Marengo, Ohio. "It's a moral dilemma for the pharmacist, knowing what would be best for the patient but not being able to help them and hoping they will ask you about the comparison."

A simple question could unlock some savings for millions of beneficiaries.

But details may be hard to find: Medicare's website and annual handbook don't mention it.

"If you don't know that there are a bunch of different prices that could be available at any given pharmacy, you don't know what you don't know," said Leigh Purvis, the AARP Public Policy Institute's director of health services research.

Related: Trump lays out his vision for lower drug prices

Researchers analyzing 9.5 million Part D prescription claims reported in a letter in the Journal of the American Medical Association in March that a patient's copayment was higher than the cash price for nearly 1 in 4 drugs purchased in 2013. For 12 of the 20 most commonly prescribed drugs, patients overpaid by more than 33%.

Although the study found that the average overpayment for a single prescription was relatively small, Newman said he had seen consumers pay as much as $30 more than the cash price.

And many beneficiaries may not know that if they pay a lower cash price for a covered drug at a pharmacy that participates in their insurance plan and then submit the proper documentation, insurers must count it toward their out-of-pocket expenses. The total of those expenses can trigger the drug coverage gap, commonly called the doughnut hole. (This year, the gap begins after the plan and beneficiary spend $3,750 and ends once the beneficiary has spent a total of $5,000.)

Daniel Nam, executive director of federal programs at America's Health Insurance Plans, a trade group, agreed that "patients should have access to the lowest price possible at the pharmacy."

But he said Medicare's warning, contained in a letter this month, takes aim at an increasingly rare occurrence. Gag order clauses are "not something they are incorporating into their contracts," he said.

UnitedHealthcare (UNH), whose popular prescription drug plans dominate the market, does not include them in any of its Medicare, Medicaid or commercial insurance contracts, said Matt Burns, a company spokesman.

Pharmacy benefit managers also said gag orders are not typical. "If it is happening, it is very much an outlier," said Mark Merritt, president and CEO of the Pharmaceutical Care Management Association, the industry's lobbying group.

Related: Check out how much Medicare spends on drugs

However, some pharmacists disagree. Kala Shankle, policy and regulatory affairs director for the National Community Pharmacists Association, which represents 22,000 independent pharmacies, said insurers have punished pharmacists who violate gag orders by dropping them from the plan's network.

In Ohio, one of several states that have banned gag orders in insurance contracts, including some Medicare drug plans, officials responded to complaints about the problem.

"The department has received inquiries related to entities withholding cost-saving information from consumers, which sometimes results in an insured [patient] paying more for pharmacy benefits than the actual cost of such pharmacy benefits," the department said last month.

Illinois and Ohio state legislators are considering bills to make such restrictions illegal, and similar legislation has been introduced in the U.S. Senate.

"If we didn't have these gag clauses, there would not be a need for the legislation and policy changes movement that's going on in the country," said Garth Reynolds, executive director of the Illinois Pharmacists Association.

Kaiser Health News (KHN) is a nonprofit news service covering health issues. It is an editorially independent program of the Kaiser Family Foundation that is not affiliated with Kaiser Permanente.