A new study links “pharmacy shopping” with many painkiller overdose deaths among patients on Medicaid. The study found patients who used four pharmacies within 90 days had the highest odds of overdosing, HealthDay reports.

Medicaid programs in many states keep track of the number of pharmacies that patients visit, in an attempt to prevent prescription drug abuse, the article notes. Pharmacy shopping can be a way for people to obtain more opioid prescription drugs than medically necessary.

The researchers examined medical records of more than 90,000 Medicaid recipients who were long-term users of opioid painkillers such as oxycodone or hydrocodone. The patients used three or more narcotic prescriptions for 90 days from 2008 to 2010. Patients who used overlapping prescriptions for painkillers were most likely to overdose. Those using four pharmacies within 90 days had the highest overdose rate.

The study is published in The Journal of Pain.

Not all patients who use multiple pharmacies intend to misuse their medication, the researchers said. Some patients may use more than one pharmacy if they move, travel or have a change in insurance coverage.

The researchers, from the Centers for Disease Control and Prevention, recommend implementing programs to restrict reimbursement for controlled prescriptions such as opioid painkillers. The programs could designate one pharmacy and one doctor for patients on these medications, they said.