Many experts say the prescription painkiller epidemic started when physicians began over-prescribing powerful opioid medications, a well-meaning attempt to more aggressively treat patients' pain. With addiction to those pills at crisis levels, they argue, a good part of the solution would be for doctors to rein in use of the drugs.

The giant health insurer Aetna is now actively encouraging change. Using the vast amount of data it collects from insurance claims by pharmacies, it has begun contacting doctors whose prescribing habits are far outside the norm.

"You have been identified as falling within the top 1 percent of opioid prescribers within your specialty," Aetna wrote to 931 physicians across the country last week. The not-so-subtle reminder was aimed at doctors who refill opioid prescriptions at very high rates compared to their peers. Only doctors who prescribed the painkillers at least 12 times were included in the data examined, which represented more than 8.6 million claims. Physicians such as oncologists, who are likely to dispense large amounts of opioids as part of their practices, were excluded.

Harold Paz, Aetna's chief medical officer, said his experience has convinced him that the best way to change doctors' behavior is to provide them with the numbers. "By nature, doctors are data-driven," he said. "It's one of the skills required to get through medical school and residency and training. It's unemotional.

"If you show them how they're doing, they'll want to do better," he said.

At the same time, Paz cautioned, Aetna can't -- and shouldn't -- assess a physician's clinical judgments with each patient. There may be legitimate reasons for a high refill rate in some cases but not others. So Aetna is urging physicians to check.

"We're asking you to look at your practice...and identify if the way you're prescribing narcotics is best practice," Paz said. "And if it's not, here's an opportunity to improve."

In 2014, U.S. physicians wrote 240 million prescriptions for narcotic painkillers, according to the Centers for Disease Control and Prevention. About 2.1 million people are addicted to legal narcotics, and 14,838 died of overdoses from opioid medications that year.

Insurers routinely send physicians data on the quality of care, which can include the number of prescriptions written, an Aetna spokesman said. But generally they do not supply the refill-to-prescription ratio, he said.

Paz said that if the 931 doctors brought their refill rate in line with the average for all physicians who prescribe opioids, 1.4 million fewer pills would be dispensed annually. The physicians receiving the letter had an average refill rate of 4.5 for each prescription written; the overall average was 0.3 refills  per prescription.

A survey published in June in the journal JAMA Internal Medicine showed that nearly 60 percent of Americans have leftover narcotics at home and 20 percent have shared them with someone.

In March, the CDC issued new guidelines for physicians who dispense opioids that encouraged them to maximize the use of non-opioid therapies, set specific criteria for stopping or continuing the use of opioids and assess a medication's effectiveness in controlling pain versus its potential harm before refilling a prescription at that dose.

The Aetna data showed that family medicine doctors (58 percent) and internists (31 percent) were by far the top refill prescribers, trailed by obstetricians (4 percent), surgeons (3 percent), neurologists (2 percent) and rheumatologists (2 percent). Pennsylvania, where the opioid epidemic has hit hard, was home to 136 "superprescribers," followed by Missouri (87), Florida (78), North Carolina (52) and Utah (45).

None of those doctors turned up on Aetna's list of doctors suspected of fraud or abuse in opioid prescribing, Paz said.

Andrew Kolodny, executive director of the advocacy group Physicians for Responsible Opioid Prescribing, said Aetna's experiment "sounds worthwhile" though probably less effective than a similar message from a government health department or law enforcement agency.

The same message should be sent to other "risky prescribers," such as physicians who authorize large doses of opioids, Kolodny said. Insurers could help more by requiring prior authorization before a doctor can order a narcotics refill, as BlueCross BlueShield of Massachusetts now does for opioid painkiller prescriptions beyond 30 days, he said.