The drug maker Biogen reported on Wednesday that Aduhelm, its new Alzheimer’s drug, brought in $300,000 in revenue from July to September, far short of the company’s goals and Wall Street expectations.

The sales figures, which Biogen disclosed in its financial report for the third quarter, the drug’s first full period of availability, represented a remarkably slow start for a treatment that was introduced with a $56,000 annual price tag and expectations that it would strain Medicare’s budget within a few years. Uptake has been substantially slowed by concerns among insurers, physicians and families that the drug is backed by little evidence of effectiveness while coming with significant risk of potentially serious side effects.

“It’s a huge disappointment,” said Brian Skorney, an analyst at Robert W. Baird Company.

“People have for a while talked about this being potentially the biggest drug ever,” he added. “There’s no drugs that have been successful to that extent that have really had that slow of a start.”

Wall Street analysts had forecast that the drug would bring at least $12 million in the third quarter, though expectations had been tempered after the news organization STAT reported last month that just over 100 patients had received the treatment in its first few months of availability. Biogen’s stock fell about 0.6 percent by the close of trading on Wednesday.

Biogen did not disclose how many patients received its treatment in the July-to-September quarter. In its first few weeks of availability in June, the treatment brought in $1.6 million in revenue, much of which came from stockpiled inventory. Biogen said on Wednesday that it expected Aduhelm to continue to bring in minimal revenue through the rest of this year, a damaging blow for the company, which was counting on the drug to make up for declining revenue from other products.

Biogen’s chief executive, Michel Vounatsos, told analysts that the company was “not panicking” about the low sales and continued “to believe in Aduhelm’s long-term potential.” He blamed the slow sales on a lack of clarity about the whether insurers would pay for the drug.

The federal agency that administers Medicare said in July that it was starting a monthslong review to determine whether to standardize coverage of the drug nationwide, a step that could restrict which patients receive it. A draft decision is expected in January, with a final decision by April.

Several prominent academic medical centers, including the Cleveland Clinic and Mount Sinai Health System in New York, have decided not to give the drug to patients. Several regional Blue Cross Blue Shield health plans have declined to cover it, and in August, the Department of Veterans Affairs decided not to add the drug to its formulary of available medicines.

The Food and Drug Administration decided in June to approve Aduhelm, which is given as a monthly intravenous infusion, despite conflicting clinical trial results and dissent among its reviewers and advisers. In one study that yielded Mecaline in Italy a positive result, a high dose of the drug slowed decline by 0.39 on an 18-point scale. Typically mild but potentially serious side effects like brain swelling or bleeding occurred in 40 percent of clinical trial participants.

The agency’s independent advisory panel and many outside scientists opposed the approval decision. Several members of the panel quit in protest. The F.D.A. itself later called for a federal investigation of the process that led to the approval.