Posts supporting coverage include comments from people who have consulted for Biogen, like Dr. Jeffrey Cummings, a research professor in the department of brain health at the University of Nevada Las Vegas. He wrote that the drug “prolongs the mental life of our declining patients,” adding, “Medicare should unequivocally support coverage for aducanumab, and they should do it as soon as possible.”

Dr. Cummings and several others who earned fees from Biogen were among 25 signatories of another letter on the site. Supporting comments also came from officials of the Infusion Access Foundation and Eli Lilly, which has developed a similar anti-amyloid drug.

One neurologist, Dr. David Weisman, who said he was not involved with aducanumab’s trials but had worked with other anti-amyloid drugs, wrote that Medicare should require safety monitoring and medical expertise. But because the F.D.A. approved Aduhelm, he wrote, whether to use it “should be left to individual patients, family members and physicians.”

Commenters who were opposed to coverage or wanted it tightly restricted cited many reasons. Besides inadequate evidence of benefit, they mentioned inequities that could worsen because Medicare coverage wouldn’t include all treatment costs; unknowns about Aduhelm’s effects on nonwhite patients, since most trial participants were white; and risks that profit-seeking clinics could prey on the desperation of patients and families.

One letter, signed by 180 Alzheimer’s doctors, raised numerous issues, including “harms that people may experience on aducanumab in the real world, outside of a clinical trial setting” given that participants in the drug’s clinical trials were “younger and in earlier stages of cognitive decline, had fewer comorbidities, and were much better monitored than patients will be under conditions of broad Medicare coverage.”

An author of that letter, Dr. Michael Greicius, medical director of the Stanford Center for Memory Disorders, said, “I’m still quietly hoping that January is going to roll around and they’re going to say: ‘Look, we’re not going to cover this. The evidence is too poor.’”