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The U.S. Food and Drug Administration on Monday approved the first ever drug to address an underlying cause of Alzheimer’s disease – an event both hailed as a medical breakthrough by millions of Americans eager for a cure and as a controversial risk by some experts concerned about the drug’s mixed clinical trial results.

Biogen’s treatment aducanumab aims to remove sticky deposits of a protein called amyloid beta from the brains of patients in the early stages of Alzheimer's - which could help stave off memory loss and other ravages of the disease.

The company said it has priced the drug, to be sold as Aduhelm, at $56,000 a year.

But some medical experts said clinical trial results were inconsistent and that more proof of the drug’s effectiveness was needed - with some trial patients experiencing potentially dangerous brain swelling.

Dr. Ronald Petersen, an Alzheimer's disease expert at the Mayo Clinic, called the drug’s approval (quote), “a big day,” adding, “but we can’t overpromise.”

The FDA in a statement said Biogen will need to conduct another trial after its approval to verify Aduhelm's benefit, adding (quote), "If the drug does not work as intended, we can take steps to remove it from the market."

Biogen has estimated that around 1.5 million Americans would be eligible for the drug, which is given by monthly infusion.

Biogen’s shares soared over 50 percent Monday on the news.