Three years ago this month, as alarms about the over-prescription of opioid painkillers were sounding across the country, the federal government issued course-correcting guidelines for primary care doctors. Prescriptions have fallen notably since then, and the Trump administration is pushing for them to drop by another third by 2021.

But in a letter to be sent to the Centers for Disease Control and Prevention on Wednesday, more than 300 medical experts, including three former White House drug czars, contend that the guidelines are harming one group of vulnerable patients: those with severe chronic pain, who may have been taking high doses of opioids for years without becoming addicted. They say the guidelines are being used as cover by insurers to deny reimbursement and by doctors to turn patients away. As a result, they say, patients who could benefit from the medications are being thrown into withdrawal and suffering renewed pain and a diminished quality of life, even to the point of suicide.

The letter writers form an uneasy alliance spanning differing positions on opioids — professors of addiction medicine as well as pain specialists, some patient representatives who have taken money from the pharmaceutical industry, and the former drug czars, from the Obama, Clinton and Nixon administrations.

Michael Botticelli, who served as the drug czar under President Obama and now leads the Grayken Center for Addiction at Boston Medical Center, said he signed the letter because “there has been enough anecdotal evidence to raise the alarm bells” about the misuse of the guidelines leading to pain patients losing effective treatment.