A tortuous history and uncertain benefits have led to controversy over its approval.

The US Food and Drug Administration (FDA) has granted conditional approval to a new drug, aducanumab (marketed as Aduhelm) for the treatment of Alzheimer’s disease.

Aducanumab is a monoclonal antibody that reduces the amount of beta-amyloid in the brain. Beta-amyloid is a protein, which many researchers believe is, at least, a partial cause of Alzheimer’s.

In early trials by the manufacturer, US biotech company Biogen, aducanumab had been shown to reduce levels of beta-amyloid in the brain but without clear-cut evidence that it slowed the progress of cognitive degeneration.

Biogen later claimed, after “a fresh look at the data”, that patients experienced “significant benefits on measures of recognition and function such as memory, orientation and language”.

The price of aducanumab is prohibitive at approximately $A72,000 per year.

Many specialists remain deeply sceptical about aducanumab and the amyloid hypothesis more broadly.

Ten out of the 11 members of an expert advisory panel appointed by the FDA voted against approving the drug on the basis of the research presented. The 11th was undecided.

Paradoxically, the incidence of Alzheimer’s in rich countries has been falling at a rate of 16 per cent per year since 1988, according to a study by the University of Harvard, thanks to improved lifestyle factors such as education and cardiovascular health.