Doctors who write many more prescriptions than their peers for potentially addictive drugs, such as opioids or stimulants, are not likely to reduce the number they write after they receive a warning from the government, a new study finds.

The study looked at prescribers who were writing many more prescriptions for potentially addictive drugs than prescribers in similar specialties who practiced nearby, Reuters reports.

“Even though we weren’t able to show that the letters were effective, this information is still useful for policymakers,” lead researcher Adam Sacarny of the Mailman School of Public Health at Columbia University said in a news release. “Based on these results, we’re now experimenting with different letter designs and making other changes to see if another approach can yield reductions in overprescribing.”

Sacarny told Reuters that previous research has found sending letters to doctors comparing them to their peers can encourage them to vaccinate their patients. He and his colleagues analyzed Medicare data from 2011-2013, and identified 1,525 doctors and other prescribers who prescribed more potentially addictive drugs than their peers.

On average, these prescribers were responsible for 406 percent more prescription drug fills than their peers. About 60 percent were general practitioners, 20 percent were nurse practitioners and 20 percent were specialists. The high prescribers were divided into two groups. One group received a letter from the Centers for Medicare and Medicaid Services, telling them how much more they were prescribing potentially addicting drugs compared with their peers.

After 90 days, the researchers found no significant change in prescribing patterns between prescribers who received the letters and those who did not. The findings are published in Health Affairs.

The researchers are planning a new study that will evaluate the effectiveness of warning letters to high prescribers of Seroquel, a prescription antipsychotic.