“What is moderate to me could be severe to you,” he said. The new language, he said, will say that the drugs should be reserved for use in patients who do not have other treatment options, and be used for management of pain that is “severe enough” to require round-the-clock treatment.

The move was prompted by a petition from Physicians for Responsible Opioid Prescribing, an advocacy group in New York. Dr. Andrew Kolodny, the president of the group, said that he was pleased by the announcement, but that he would have liked the agency to have gone further, for example by defining limits on the duration of use and dosage.

A major issue hanging over opioid use is a lack of data about the drugs’ long-term risks and effectiveness. Dr. Throckmorton said the agency was trying to address that by telling producers that they had to run studies and clinical trials on drugs currently on the market to better assess the risks. In a letter sent to drug makers, the agency said that companies would also have to perform other studies, including one examining whether patients on opioids develop increased sensitivity to pain.

The next step, Dr. Throckmorton said, is “a series of conversations” with manufacturers of such drugs “about what those studies and trials will look like.” He expects to start receiving information in 2015.

Dr. Erin E. Krebs, a pain researcher at a Department of Veterans Affairs facility in Minneapolis, said that “it would be fabulous” to have such data so that doctors could better understand how or whether they should be prescribing opioids for long periods.