Doctors who prescribe large amounts of opioids are receiving big payments from drug makers, according to an analysis by CNN and researchers at Harvard University. The more opioids doctors prescribe, the more money they receive.

Hundreds of doctors received six-figure sums in 2014 and 2015 for speaking, consulting and other services from drug manufacturers. Thousands more were paid more than $25,000. Doctors who prescribed the largest amounts of opioids were the most likely to receive payment, the analysis concludes.

The researchers said it is not clear whether the payments entice doctors to prescribe more opioids, or whether the drug companies reward doctors who are already prescribing large amounts of the drugs.

“I don’t know if the money is causing the prescribing or the prescribing led to the money, but in either case, it’s potentially a vicious cycle. It’s cementing the idea for these physicians that prescribing this many opioids is creating value,” said Dr. Michael Barnett, assistant professor of health policy and management at the Harvard T.H. Chan School of Public Health.