– A new study offers some of the strongest evidence yet of the connection between the marketing of opioids to doctors and the nation’s addiction epidemic.

It found that counties where opioid manufacturers offered a large number of gifts and payments to doctors had more overdose deaths involving the drugs than counties where direct-to-physician marketing was less aggressive.

The study, published Friday in JAMA Network Open, said the industry spent about $40 million promoting opioid medications to nearly 68,000 doctors from 2013 through 2015, including by paying for meals, trips and consulting fees. And it found that for every three additional payments that companies made to doctors per 100,000 people in a county, overdose deaths involving prescription opioids there a year later were 18 percent higher.

From 2013 through 2015, roughly 1 in 12 doctors received opioid-related marketing, the study said, including 1 in 5 family practice doctors.

The authors, from Boston Medical Center and New York University School of Medicine, found that counties where doctors received more industry marketing subsequently saw an increase in both the number of opioids prescribed and opioid-related overdose deaths.

Dr. John Cullen, president of the American Academy of Family Physicians, said, “A limitation of the study, as acknowledged by the authors, is the many unknown variables that prevent drawing a direct causal link between pharmaceutical marketing and opioid-related deaths.”

The authors acknowledged that the study could not differentiate between overdose deaths involving painkillers that are prescribed vs. illicitly acquired.

“We acknowledge that our work describes only one part of the very complex opioid overdose crisis in this country,” said the lead author, Dr. Scott Hadland. “Even still, prescription opioids remain involved in one-third of all opioid overdose deaths, and are commonly the first medications that people encounter before transitioning to heroin or fentanyl.”

Areas with large numbers of payments and high overdose rates included four cities in Virginia as well as Cabell County, W.Va., which has one of the highest overdose death rates in the nation. Lackawanna County, Pa., also ranked high in both measures, as did Erie County, Ohio.