Merck & Co.’s decision this week to stop a clinical study of its once-promising Alzheimer’s disease drug — just three months after an Eli Lilly & Co. treatment failed in a trial — is likely to focus even more attention on Biogen Inc.’s experimental medicine for the mind-ravaging condition. But it also raises concerns that an effective treatment will continue to remain elusive for years to come.

Merck ended its trial of the drug after an outside panel of medical specialists concluded there was “virtually no chance of finding a positive clinical ­effect.’’ A separate trial involving ­patients who are showing only early symptoms of the degenerative disease will continue, Merck said, since the panel didn’t find safety signals worrisome enough to also halt that test.

“It’s very disappointing, once again,’’ said David Knopman, a professor of neurology at the Mayo Clinic in Rochester, Minn.

Alzheimer’s affects about 44 million people worldwide, a number that is sharply rising as baby boomers age. It’s been more than a decade since a drug to lessen the disease’s symptoms came on the market, and none halt its progression.

More than a dozen companies have Alzheimer’s drugs in late-stage clinical trials — including Cambridge-based Biogen, Takeda Pharmaceutical Co., and the Genentech division of Roche AG — according to the Alzheimer’s Association.

Merck’s drug, like Biogen’s and others, targets amyloid plaque in the brain. The plaque is believed to be a cause of the disease.

“There is mounting evidence — of which this is another piece — that removing amyloid once people have established dementia is closing the barn door after the cows have left,’’ Knopman said.

Michel Vounatsos, Biogen’s new chief executive, has said that pinpointing a treatment for Alzheimer’s is the biotech’s “number one challenge and opportunity.’’ Findings from an early-stage trial of the company’s Alzheimer’s drug, called aducanumab, were promising. In December, Biogen said the drug kept lowering amyloid plaque levels in patients who continued to take it after the initial round of testing.

The company also said that incremental increases in the doses given to a new set of about two dozen patients reduced the percentage of those who had brain swelling, compared with those given only the highest dose in an earlier part of the study.

Aducanumab also reduced amyloid plaque in the new set of patients, compared with a smaller group of new patients who were given a placebo.

Biogen is now in the process of enrolling patients in a more advanced trial that will involve up to 3,000 people in more than 20 countries. It’s likely to take a year or more for the first data to be reported.

In the Merck trial, it’s possible that the disease may have advanced too far in patients for the drug to have any tangible effect. Verubecestat and drugs like it, called BACE inhibitors, operate before the amyloid has formed into plaque. In the trial, patients’ brains already would have had plaque buildup.

“You need to go in at the very earliest stage of disease to show the kind of improvement we are hoping for,’’ said James Hendrix, director of global ­science initiatives at the Alzheimer’s Association.

It’s also possible that the Merck drug was too weak, or that not enough of the medicine was making it into the brain, Knopman said. Previous studies have shown that the drug does have a powerful impact on amyloid levels, though doctors and others in the ­industry will have to wait for more ­details of the trial to draw any ­conclusions, he said.

Mark Pothier of the Globe staff contributed to this report. Material from Globe wire services also was used. Pothier can be reached at mark.pothier@globe.com. Follow him on Twitter @markpothier.