Dr. Stefan G. Kertesz, who teaches addiction medicine at the University of Alabama at Birmingham, submitted a letter in opposition, signed by 220 professors in academic medicine, experts in addiction treatment and pain management, and patient advocacy groups.

His patients include formerly homeless veterans, many of whom have a constellation of physical and mental health challenges, and struggle with opioid dependence. For them, he said, tapering opioids does not equate with health improvement; on the contrary, he said, some patients contemplate suicide at the prospect of suddenly being plunged into withdrawal.

“A lot of the opioid dose escalation between 2006 and 2011 was terribly ill advised,” Dr. Kertesz said. “But every week I’m trying to mitigate the trauma that results when patients are taken off opioids by clinicians who feel scared. There are superb doctors who taper as part of a consensual process that involves setting up a true care plan. But this isn’t it.”

Some two dozen states and a host of private insurers have already put limits on opioids, and Medicare has been under pressure to do something, too. Last July, a report by the inspector general at the Department of Health and Human Services raised concerns about “extreme use and questionable prescribing” of opioids to Medicare recipients. In November, a report from the Government Accountability Office took Medicare to task, urging greater oversight of opioid prescriptions.

If the rule takes effect, Mark Zobrosky’s experience could be a harbinger for many patients. Mr. Zobrosky, 63, who lives in the North Carolina Piedmont, takes opioids for back pain, which persists despite five surgeries and innumerable alternative treatments. He has an implanted spinal cord stimulator that sandpapers the edge off agony, and has broken four molars from grinding because of pain, he said. He receives Medicare as a result of his disability, including a private plan that pays for his drugs.

He submits to random urine tests and brings his opioids to his doctor to be counted every month. To prepare for mandatory reductions, his doctor has tapered him down to a daily dose equivalent of about 200 milligrams of morphine. (Mr. Zobrosky has a large frame; doctors say that opioid tolerance depends on many factors — one person’s 30 milligrams is another person’s 90.)