Erika Marble visits the gravesite of Edward Martin III, her fiance and father of her two children, in Littleton, N.H. The 28-year old died Nov. 30, 2014, from an overdose of the opioid Fentanyl. (Jim Cole/AP)

Of the 98 million Americans who take prescription opioids each year, almost 3 million receive their prescriptions from five to as many as 20 different physicians.

The practice, often called “doctor shopping,” allows opioid-addicted individuals to consume an enormous and dangerous number of pills with the unwitting help of doctors. Doctor shopping also lets drug dealers use the health care system to subsidize their business.

Fortunately, state Medicaid programs have developed a simple fiscal policy that reduces the problem – and it could even more effective if adopted across all insurers.

Doctor shopping is possible because none of the doctor shoppers’ prescribers knows that they are only one of many providers of opioids. Enter someone with full information: the insurer. Medicaid programs for example can examine all the separate billings and recognize that an enrollee is probably a doctor shopper. This allows the insurer to respond through “reimbursement lock-in,” meaning that the enrollee’s prescriptions for opioids will only be covered if they are written by a single provider of the patient’s choosing.

Evaluations of Medicaid lock-in programs generally show they decrease prescribing of opioids as well as other controlled substances, such as benzodiazepines. North Carolina’s program for example, reduced controlled substance prescriptions by 17 percent among Medicaid enrollees who had histories of unusually large numbers of prescriptions and prescribers.

Reducing prescriptions reduces costs to the payer, who in the case of Medicaid is ultimately the taxpayer. Locking patients with many prescriptions into single providers should also benefit patients by reducing their risk of overdose. However, in unusual situations lock-in could pose difficulties. For example, if patients suffered injuries while on vacation and needed an opioid prescription, the doctor who treats them may have difficulty reaching the approved prescriber back home.

Reimbursement lock-in programs can be evaded. Some individuals make cash purchases to surmount them. Other individuals covered by more than one type of insurance (e.g., the more than nine million individuals covered both by Medicare and Medicaid) may simply shift some of their prescription reimbursement enough to stay below lock-in thresholds. This tactic will become harder in 2019 when Medicare is slated to adopt reimbursement lock-in programs, and would become harder still if all private insurers followed suit.