In a landmark move, the US Food and Drug Administration (FDA) on 7 June, approved a new Alzheimer's drug, Aduhelm.

This is the first drug for the treatment of Alzheimer's disease to be approved by the US FDA in nearly 20 years, but the decision has been rife with controversy, with many health experts questioning its soundness.

So far, two members of the FDA's advisory committee have resigned over the decision to approve the treatment despite the advisory recommending against it, reported the Washington Post.

What do we know about this drug? Why has it not gone down well with health experts, and what is the controversy surrounding it? FIT breaks it down.

First, let's look at what the treatment is, and what its clinical trials say.

The new drug, Aduhelm, manufactured by pharmaceutical company, Biogen Inc. is a treatment for Alzheimer's disease.

First, it must be clarified that this drug is not a cure for Alzheimer's, nor is it a miracle drug that can reverse the progression of the disease.

According to Biogen, clinical trails of Aduhelm were conducted "in patients with early stages of Alzheimer’s disease (mild cognitive impairment and mild dementia) with confirmed presence of amyloid pathology."

According to the Alzheimer's Association, beta-amyloids are sticky substances that build up in the brain and form plaques.

These are thought to start forming in the brain sometime before the physical symptoms of Alzheimer's show up and can be used to identify early onset cases.

Moreover, the drug was tested only on patients in very early stages of the disorder, and there is no evidence of the drug working in patients who are in moderate to advanced stages.

An advisory of 11 members nearly unanimously voted against the approval of this drug on the grounds of inconclusive evidence.

However, on Monday, 7 June, going against the advisory's recommendation, the FDA gave the drug "accelerated approval".

The FDA, in a second statement, addressed the controversy surrounding the approval and justified it saying, the decision was made based on clinical trials that showed a reduction in amyloid-beta plaques in the brain which is thought to be the physical markers of Alzheimer's disease.

Interestingly the FDA rejected Biogen's application for approval of the drug back in November.

The FDA's approval, further comes with the condition of a 4th clinical trial to prove that the the drug does, in fact, have actual long term benefits.

The 'accelerated approval pathway' that the FDA used to greenlight this drug is meant for treatments for life threatening illnesses "that provide a meaningful therapeutic advantage over existing treatments."

Something, that health experts argue, that this drug doesn't provide.

The hasty approval of the drug without sufficient evidence that it works is something scientists are divided on.

“I consider the approval of aducanumab represents a grave error that will have only negative impact on patients and their families and that could derail the ongoing search for meaningful dementia treatments for a decade," the Guardian quoted Dr Robert Howard, Professor of Old Age Psychiatry at UCL.

Critiques also point to how expensive the treatment is.

They worry is that this could lead desperate families to spending thousands of dollars on a drug that may not work.

Around 6.2 million people in the US alone suffer from this disease and critiques talk of the false ray of hope it provides to families who expected the drug might 'bring back their parents'.

(Written with inputs from Reuters, and The Washingtom Post, the Guardian)