"The CHMP recommended not granting a marketing authorisation for Leqembi, a medicine intended for the treatment of Alzheimer's disease," the European Medicines Agency said, referring to its committee for evaluating drugs for human use.

Leqembi uses an active substance called lecanemab to treat adults with mild memory and cognitive problems resulting from the early stages of the common type of dementia.

But the CHMP said "the observed effect of Leqembi on delaying cognitive decline does not counterbalance the risk of serious side events associated with the medicine".

"The most important safety concern with Leqembi is the frequent occurrence of amyloid-related imaging abnormalities (ARIA), a side effect, seen in brain imaging, that involves swelling and potential bleedings in the brain", the Amsterdam-based EMA said.

Leqembi is a monoclonal antibody, a type of protein that clings to a substance in the brain and can delay worsening of the disease.

Leqembi, together with another Alzheimer's drug called Aduhelm, received approval from the US Food and Drug Administration early last year.

Both drugs were approved through an accelerated process by the FDA for drugs treating serious conditions where there is an unmet medical need.

Around eight million people in the European Union live with dementia, with Alzheimer's disease accounting for more than half of these cases, according to the Alzheimer Europe website.