Shares of Biogen slid Wednesday, a day after federal regulators slapped coverage limitations on the drugmaker's new Alzheimer's disease treatment.

The Centers for Medicare and Medicaid Services said Tuesday after markets closed that patients taking Biogen's Aduhelm will have to also participate in clinical trials to assess the drug's effectiveness in order for Medicare to cover the cost.

That's an important caveat because Medicare is expected to cover most of the patients who opt for Aduhelm, and the drug can cost as much as $28,000 annually, not counting expenses for brain scans and other care patients will need while taking it.

Biogen had cut that cost down from more than $50,000 annually last month after taking months of criticism over the expense.

The initial Medicare coverage decision will significantly limit access to Aduhelm, said Mizuho Securities USA analyst Salim Syed. He had previously estimated around $3.5 billion to $4 billion in peak annual sales for the drug.

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 more than 8%, or nearly $21, to $220.56 Wednesday morning while broader indexes rose slightly at the start of trading.

The stock price had soared past $400 after the FDA decision in June.

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Follow Tom Murphy on Twitter at @thpmurphy