After months of wait, Biogen Inc. BIIB has finally achieved success by taking its controversial Alzheimer's treatment past the finish line.

FDA Okays Aducanumab: Biogen's biologics license application for aducanumab, its investigational Alzheimer's treatment, received FDA approval under the accelerated approval pathway.

Aducanumab, which will go with the tradename Aduhelm, has the distinction of being the first Alzheimer's treatment approved since 2003.

More importantly, it's the first treatment directed at the underlying pathophysiology of Alzheimer's disease, the presence of amyloid beta plaques in the brain, the FDA said in a statement. Aduhelm works by reducing plaques and will also lead to a reduction in the clinical decline of dementia.

The FDA conceded that data included in Biogen's submission were highly complex and left residual uncertainties regarding a clinical benefit.

"At the end of the day, we followed our usual course of action when making regulatory decisions in situations where the data are not straightforward," the agency said.

The FDA said it took into account clinical trial findings, Adcom's input, and perspectives of the patient community, and concluded that the benefits of Aduhelm for patients with Alzheimer's disease outweighed the risks of the therapy.

The regulator also highlighted the acute need for a therapy for Alzheimer's, given more than 6 million Americans are living with Alzheimer's disease and the disease is the sixth leading cause of death in the U.S. This number is expected to grow as the population ages.

Biogen's Long-Winding Route to Approval: After giving up on aducanumab due to not-so-promising late-stage study results, Biogen revived it following a subset analysis of data that ultimately convinced the FDA to accept a regulatory filing for approval.

Aducanumab was subsequently put to Adcom test but could not win the backing of the FDA panel.

Even after the adverse outcome, the sell-side was hopeful that aducanumab will successfully clear the FDA hurdle. Oppenheimer analyst Jay Olson, who rates Biogen a Buy with a $325 price target, said in a note in mid-May he sees equal probabilities of approval and rejection.

Following the Adcom meeting, the FDA announced a three-month delay in decision from March 7 to June 7.

What's Next? The FDA said it will continue to monitor Aduhelm as it reaches the market.

Additionally, FDA said it has required Biogen to conduct a post-approval clinical trial to verify the drug's clinical benefit: "If the drug does not work as intended, we can take steps to remove it from the market."

At the time of publication, Biogen's stock was halted for trading at $286.43 a share.