Pfizer kept its finding under wraps for more than three years because, it claims, it didn't believe the evidence was strong enough.

It found the link between Alzheimer's and the drug Enbrel when analysing medical insurance claims in hundreds of thousands of people in the US.

People taking Enbrel, an anti-inflammatory used to treat rheumatoid arthritis, appeared to be 64 per cent less likely to develop the memory-robbing disorder.

But after years of discussion, the company decided not to reveal what it had found, according to a document seen by the Washington Post.

Experts are sceptical of the firm's reasons not to share its 'strong' data at a time when scientists are deep into a quest to find an Alzheimer's treatment.

One suggested money – and a desire to not let others capitalise – may have been behind the decision because Enbrel's patent had almost run out.