For Phil Gutis, 58, a former New York Times reporter diagnosed with early onset Alzheimer’s, “it was a kick in the stomach.” Like Dr. Poritsky, he was enrolled in the aducanumab trial, and had learned of its termination from a friend’s text. “There should be a better way,” he said.

Internationally, the Alzheimer’s Association calculates that clinical trials now underway for dementia treatments — drugs, dietary programs, devices and other interventions — aim to enroll more than 56,000 people.

Drug trials for Alzheimer’s disease are often a particularly arduous commitment.

“These are not simple protocols,” said Dr. Sharon Cohen, a neurologist and principal investigator at the Toronto Memory Program, which had enrolled 29 participants in the aducanumab trial. “The visits are long. They are frequent. There is in-depth testing. Blood draws. M.R.I. scans that may recur. PET scans. There may be spinal taps. And the study partner” — a family member or friend — “has to attend many of these as well.”

Why agree to all that, especially when researchers pointedly explain that the experimental drug may not help and could actually harm patients? Moreover, in a typical double-blind study, half the participants won’t even get the drug but will instead receive a placebo.

Researchers and patients describe a mix of motives: desperation, altruism, trust in the investigators and sponsors.

“I thought I was doing this for future generations,” Mr. Gutis said. But as he learned more about aducanumab, “there was definitely optimism that this possibly could help me.”

Participants also come to value their deepening relationships with the study staff, who know so much about the disease and take such an interest in their condition. “It was almost familial,” Dr. Poritsky said.