Doctors who receive gifts and payments from opioid manufacturers are more likely to prescribe higher amounts of opioid painkillers than their colleagues within a year, researchers at the University of Pittsburgh have found.

The study, published Tuesday in the Journal of General Internal Medicine, analyzed prescribing data from Medicare Part D records and payments from pharmaceutical companies to doctors for promotional speaking, consulting, lunches, and other expenses, which the companies are required by federal law to report publicly.

While several previous studies have focused on the relationship between pharmaceutical promotions and doctors’ prescribing habits, the new study, conducted by researchers in Pitt’s department of health policy and management, was the first to focus on how those payments affect physicians across different specialties.

Researchers found that, compared with doctors who received no pharma money, psychiatrists and neurologists who were paid more than $100 by pharmaceutical companies were seven to 13 times more likely to prescribe more opioids than nearly all of their colleagues in the following year. Primary care physicians were 3.5 times more likely to prescribe higher amounts of opioids if they were paid $100 or more, the study found.

Among doctors in other specialties, like hematology, the influence of opioid marketing on prescribing habits wasn’t as stark. But even receiving small payments from a pharmaceutical company made doctors more likely to increase their opioid prescribing in the following year, the study found.

“There’s definitely some behavioral science and social science literature that indicates that providers may perceive themselves immune to bias from gift giving — we all may perceive ourselves immune to bias from gift giving,” said Mara Hollander, the study’s lead author and a doctoral student at Pitt. “But all gifts have been shown to create an unconscious bias toward prescribing.”

And, the study notes, the marketing of opioid painkillers in particular can have alarming effects on the health of a population. A study published in January in the Journal of the American Medical Association found that counties where opioids had been marketed to physicians saw higher rates of opioid overdose deaths.

In the time period analyzed in the Pitt study — 2014 to 2016 — 18 pharmaceutical companies reported that they had promoted opioid products to doctors with gifts and other payments, Hollander said.

Just a handful of companies were responsible for the bulk of the payments: Nearly half were from Insys, which sells a name-brand fentanyl called Subsys. An additional 22% of the payments were made by Purdue Pharmaceuticals, whose intense marketing of OxyContin painkillers has been the target of federal lawsuits and public ire.

Purdue has since stopped marketing opioids to doctors entirely.

“There’s still a huge number of people living with opioid use disorder in this country, and we still have high rates of opioid prescribing,” even though prescribing has dropped in recent years, Hollander said. “We should be considering why they’re still high. The relationship between pharmaceutical companies and physicians and their prescribing still exists, and definitely deserves a look from physicians and policymakers.”