Insurance companies may be asking people to shell out more money for drug co-payments than the drugs actually cost, a new study suggests.

Generic drug co-payments in the U.S. exceeded the cost of medicines about 28 percent of the time – or for more than one in four prescriptions, researchers found.

Co-payments for branded drugs were higher than the medication cost about 6 percent of the time, they report in JAMA.

“This is money that patients could be saving if they knew about and could avoid the practice,” said lead study author Karen Van Nuys of the Schaeffer Center for Health Policy and Economics at the University of Southern California in Los Angeles.

While some people might not notice the overpayment because their medicines cost only $5 or $10 with insurance, added costs at the drugstore could make it even harder for some low-income patients to afford their medicines and might lead them to skip doses, delay refills or stop taking drugs altogether.

“Particularly for patients who struggle to afford their prescription drugs, even modest savings would be helpful,” Van Nuys said by email.

For the study, researchers examined 2013 data on the average prices retail pharmacies get paid for filling prescriptions for people with private health insurance as well as the co-payments, or out-of-pocket fees, patients paid for their medicines.

$135 Million in Overpayments

Out of 9.5 million claims for prescription drugs, 2.2 million involved overpayments, the study found.

Altogether, the overpayments totaled $135 million, or about $10.51 per person.

The average overpayment was $7.69. Overpayments exceeded $10 in about 17 percent of claims.

For brand-name drugs, overpayments weren’t as common, but they were higher on average at $13.46, compared with average overpayments of $7.32 for generic drugs.

The most commonly prescribed drug, the painkiller hydrocodone with acetaminophen, involved overpayment in one-third of cases, with an average overpayment of $6.94.

Twelve of the 20 most commonly prescribed drugs involved overpayment rates above 33 percent.

Limitations of the study include the potential for pricing in 2013 to be different than what it would be today, the authors note.

It’s also important to recognize that using generic drugs can offer patients effective treatments at lower costs than similar brand-name alternatives, said Dr. Aaron Kesselheim, a researcher at Harvard Medical School in Boston who wasn’t involved in the study.

While previous research has linked high drug costs to an increased likelihood that poor people will stop taking medicines, the amount of the overpayments found in the current study might not be large enough to change patient behavior, Kesselheim said by email.

Cost-related non-adherence is usually more of an issue when there’s a choice “between extremely inexpensive generic drugs and extremely expensive brand-name drugs, not on average about $5 differences between co-payments and negotiated reimbursement for generic drugs,” Kesselheim said.

Consider Paying Cash

To avoid overpayments, patients should always ask the pharmacist if their costs would be lower if they paid cash instead of using their insurance, said Stacie Dusetzina, a health policy researcher at Vanderbilt University Medical Center in Nashville, Tennessee.

“Pharmacists might not be allowed to offer this information to patients due to `gag clauses’ but if patients ask, pharmacists can tell them,” Dusetzina, who wasn’t involved in the study, said by email.

“Several states have banned these practices and allow pharmacists to offer this information but even if you live in one of those states, you should ask,” Dusetzina added. “Patients can also use pricing tools on the internet like GoodRx.com to see what prices they could expect across a variety of pharmacies if they paid cash.”