Why Biogen's Alzheimer's drug Aduhelm is so controversial
The U.S. Food and Drug Administration's approval of Biogen 's Alzheimer's drug Aduhelm in June was heralded as a historic triumph in the fight against a disease that kills thousands of Americans every year.

But the excitement around the first FDA-approved drug to target the underlying cause of the memory-robbing disease—not just its symptoms—has ebbed since then. The drugmaker reported just $300,000 in revenue from Aduhelm sales during the third quarter, a fraction of what Wall Street was expecting.

Physicians have been split over whether clinical trial data proves the monoclonal antibody actually slows cognitive decline. What's more, Medicare is debating whether to cover seniors on the government-run insurance program for Alzheimer's treatments, a decision that is crucial to their profit potential.

The Centers for Medicare and Medicaid Services plans to issue a draft decision by mid-January, a spokesperson told CNBC.

Meanwhile, shares of the Cambridge, Massachusetts-based biotech have cratered roughly 40% since the beginning of June. Given Aduhelm's lackluster debut, as well as generic competition for Biogen's other drugs, the company now plans to cut $500 million in annual costs.

In December, it also said it would slash Aduhelm's controversial $56,000 annual list price by about 50% in a bid to boost sales. The company anticipates 50,000 new patients could start Aduhelm in 2022 with insurance coverage and greater access to diagnostics and specialized centers.

"In terms of this new drug, we are keeping an open mind," said Dick Novik, who retired from the broadcast business to help care for his wife, Eugenia Zukerman, following her Alzheimer's diagnosis.

"If only there were a way of being more certain in our minds that it works," Novik told CNBC.

Why is Biogen's Alzheimer's drug so controversial? Watch the video above to learn.