The United States is currently in the midst of a prescription painkiller overdose epidemic. According to the Centers for Disease Control and Prevention (CDC), 44 Americans die from prescription painkiller overdoses each day. That said, understanding what leads to these overdoses will inform health care providers of the best ways to prevent them. A new study finds one of the ways people get their hands on more opioid painkillers than they need is by visiting multiple pharmacies to fill prescriptions.

The study, from researchers at the CDC and a number of other organizations, looked at the prescription opioid-buying behaviors among more than 90,000 Medicaid enrollees aged 18 to 64 in Washington State. It found that people who visited four or more pharmacies to fill their prescriptions over the course of 90 days were the most likely to overdose on the powerful pain medications. What’s more, those who had overlapping prescriptions were the most at risk of overdosing, according to a press release.

The researchers wrote that the “first step in implementing targeted prevention policies” is identifying those who are able to get their hands on more prescription opioids, such as Vicodin and OxyContin, than they need. Visiting multiple pharmacies to buy prescription painkillers is known as pharmacy shopping, while obtaining multiple prescriptions for a drug is known as doctor shopping. But while both are illegal, it’s often hard to tell if a person is trying to obtain the drugs illegally, or if they’ve simply moved to another address or had changes in insurance coverage, among other factors.

Other studies have found pharmacy and doctor shopping to be a problem elsewhere, too. In 2012, for example, a study looking at data from the West Virginia Controlled Substances Monitoring Program found that more people who had died were likely to be pharmacy shoppers and doctor shoppers than those who were still living — 25.2 percent of dead subjects were pharmacy shoppers compared to 3.6 percent of living subjects. West Virginia is also among the states with the highest rates of prescription opioid overdoses.

The authors of the current study said factors like moving and changes to insurance coverage could be accounted for through patient review and restriction programs. They also suggested restricting Medicaid reimbursement for prescription opioids to only one designated physician and pharmacy. These measures would work in conjunction with already in-place measures, such as prescription drug monitoring programs, which is essentially an electronic database that tracks opioid prescriptions, patients at risk of overdose, and insurance fraud.

Source: Yang Z, Wilsey B, Bohm M, et al. Defining Risk of Prescription Opioid Overdose: Pharmacy Shopping and Overlapping Prescriptions Among Long-Term Opioid Users in Medicaid. The Journal of Pain. 2015.