But the company changed the design of the second study midway through in order to give one group of patients a higher dose. As of March, the change had brought no clinical benefit. But as more data came in, officials say, the picture changed.

Biogen now has data from twice as many patients, from both clinical trials, and executives used those results to justify seeking approval. But the findings have not been published in a peer-reviewed journal, and experts have plenty of questions about whether aducanumab actually works, and if so, how well.

The company plans to present its analyses at a scientific meeting in December.

If the drug was deemed a failure, why are we talking about it?

Some 5.5 million Americans have Alzheimer’s disease, not including those in the very early stages, and there is no effective treatment. The consequences for patients and caregiving families are devastating.

Biogen officials contend the additional study data showed a slower cognitive decline in the subgroup of patients who got the highest dose. That finding, plus the somewhat positive results from the other study, may be enough to convince the F. D.A. that the drug works.

Biogen met with agency officials on Monday, and they told the company that it was “reasonable to file” for approval, Mr. Vounatsos said.

An F.D.A. spokesman said the agency does not comment on investigational drugs or on drug approval applications.

Why the hesitation?

First, Biogen has provided only a summary of its data and analyses. When Alzheimer’s specialists see the details in December — better yet, when there is a peer-reviewed published paper — they may not be convinced the drug has meaningful benefits.