Melbourne: If you smoke and think that it prevents deadly Alzheimer's disease, you are certainly wrong. Smoking prevents Alzheimer's disease is a myth, a new study has claimed.

In fact, after reviewing more than 40 research papers published since 1984, experts at the University of California have said that smoking nearly doubles the risk of developing the disease.

Professor Jurgen Gotz, from the University of Sydney's Brain and Mind Research Institute, pointed out that industry-linked studies could have been influenced.

"There have been many studies looking at the incidence of Alzheimer's disease in general, and dementia in general, and the role of nicotine," News.com.au quoted him as saying.

He added: "Some of these studies showed, or claimed to show, that smoking, in a sense, protects from Alzheimer's disease.

"It turns out when one takes these (industry-linked) studies into consideration ... the bottom line is smoking indeed is associated with an increased risk of Alzheimer's disease."

He concluded: "This has been disproved both in humans, as studies show, and in animals and cell culture systems.

"Nicotine ... increases the degenerative changes in the brain."

Studies into tobacco and Alzheimer's had begun in the late 1970s, after reports came of lower rates of the condition among older smokers.