But he said Wednesday he now assumes less than $100 million.
Medicare’s national coverage determination will become final by April 11, following a public comment period and further evaluation by the agency.
The U.S. Food and Drug Administration had approved Aduhelm last June and later said it was appropriate for patients with mild symptoms or early-stage Alzheimer’s.
Aduhelm clears brain plaque thought to play a role in Alzheimer’s disease, and regulators made their call based on research showing the drug seemed likely to benefit patients. But they’ve asked for another study.
Biogen said last month that it would submit final plans for that study to the Food and Drug Administration in March. It then plans to have the first patient screened for the research by May.
Researchers will aim to enroll about 1,300 people with early-stage Alzheimer’s and expect to complete the research about four years after the study begins.
Aduhelm has been hailed as a promising treatment by some researchers because it has the potential to slow the fatal Alzheimer’s. No drugs on the market currently do that.
But concerns about the cost and effectiveness have slowed the drug's debut.
Biogen CEO Michel Vounatsos said on Monday that the company now has about 220 sites treating patients with Aduhelm.
Biogen said in June, shortly after the FDA approval, that about 900 sites in the U.S. have the equipment and expertise to immediately begin giving the drug, which requires monthly IVs.
Shares of Cambridge, Massachusetts-based Biogen Inc. fell $16.18, or 6.7%, to close Wednesday at $225.34.
The stock had soared past $400 after the FDA decision in June.
Follow Tom Murphy on Twitter at @thpmurphy