The US Food and Drug Administration (FDA) approved on Monday a controversial new drug to treat Alzheimer’s Disease, the first in nearly 20 years.

Aducanumab, also known as Aduhelm, attempts to fight the disease in its early stages by targeting amyloid beta plaques in the brain to slow the disease's progression.

The FDA's decision to grant what is known as Accelerated Approval comes after its own external experts concluded last year that there is insufficient evidence to support the drug's effectiveness.

The health agency recognized the fact multiple times in announcing its decision, saying "the data included in the applicant’s submission were highly complex and left residual uncertainties regarding clinical benefit."

"There has been considerable public debate on whether Aduhelm should be approved. As is often the case when it comes to interpreting scientific data, the expert community has offered differing perspectives," the FDA said in a statement.

"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," it added.

It said drugmaker Biogen must conduct "post-approval studies to verify the anticipated clinical benefit," noting the FDA "has regulatory procedures in place that could lead to removing the drug from the market."

The Alzheimer's Association non-profit lauded the FDA's decision to grant approval, saying "we celebrate today’s historic decision."

"This approval is a victory for people living with Alzheimer's and their families," association president Harry Johns said in a statement./aa