Biogen said more than 900 infusion centers will be ready to administer the treatment.

It can't come sooner for the more than 6 million Americans living with Alzheimers and the first approved treatment in 18 years.

Ronald Petersen heads the Mayo Clinic's Alzheimer's Disease Research Center.

"I think this is an exciting day for people with Alzheimer's disease. They now have some hope that a drug is going to be on the market that is going to have an impact on the underlying disease process itself."

While patient advocates and some neurologists hailed the drug, others said the clinical trial results were inconsistent and more testing was needed on the drug's effectiveness. Still, it could soon be a boon to Biogen, which says revenue from Aduhelm could start ramping up next year.

Analysts at Guggenheim estimate the drug - which will cost patients $56,000 per year - could generate over $1 billion in sales in 2022.

But Brown University Medical School Neurology Professor Stephen Salloway hopes competition will lower that price tag.

"Hopefully this is the first, and there'll be more to come, and then there'll be competition, and hopefully competition will lead to better pricing. And then hopefully there'll be other treatments that we can discover that are more economical."

Most of the cost will be covered by U.S. health insurers and the government's Medicare program.

The company's shares soared 38% Monday after the Food and Drug Administration approved Adulhelm as the first ever drug to target a likely cause of Alzheimer's disease. The shares gave back some of those gains in early trading Tuesday even though a number of brokerage analysts upgraded the stock and raised their price targets.