The Food and Drug Administration of the US on Monday approved Biogen's Alzheimer's drug, aducanumab, the first treatment for the disease in nearly 20 years. Aducanumab has been approved despite the controversy over the mixed results of its clinical trials.

"Aduhelm is the first treatment directed at the underlying pathophysiology of Alzheimer's disease, the presence of amyloid-beta plaques in the brain," said the FDA's Patrizia Cavazzoni.

Aducanumab, which includes a long list of drugs, is aimed at removing sticky deposits of amyloid-beta from the brain of Alzheimer's patients in the early stages of the disease. The drugs will help the brain to stave off Alzheimer's ravages that include memory loss and self-care ability. 

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What is the controversy around the Biogen's drug?

While some patient advocates and neurologists have hailed Aducanumab, other doctors say the results of clinical trials of the Biogen drug were inconsistent and more proof was needed. 

The FDA's approval goes against an independent expert panel's recommendation last November said the evidence of the drug's benefit is insufficient. 

Tested in two late-stage human trials known as Phase 3 trials, Aduhelm, which is given by monthly infusion, showed a reduction in cognitive decline in one, but not the other.

Meanwhile, some patients experience potentially dangerous brain swelling.

The results have raised concerns as Biogen has estimated that around 1.5 million Americans would be eligible for treatment with aducanumab.