The finding, from a study of almost 7,000 men and women, is surprising as previous research has suggested that smoking provided some protection against dementia.
In the new study, which is larger than earlier ones, doctors in the Netherlands selected men and women without dementia aged 55 or older. After two years, 146 had developed dementias of all kinds, of whom 105 had Alzheimer's disease which was confirmed by neuropsychological assessment and, in some cases, a brain scan.
In addition to the doubled risk of both dementia and Alzheimer's disease, smokers also tended to develop dementia at a younger age.
The researchers, who report their findings in the Lancet medical journal, found that smokers who carry the Apoe 4 gene, which has been linked with Alzheimer's, were at no greater risk of developing the disease than non- smokers. However, those without the gene were at four times the risk.
One explanation of why Apoe 4 might protect smokers is that smoking alters brain chemistry in a way that counters some of the effects of Alzheimer's disease. However, it is also possible that smokers with Apoe 4 die young, so do not live long enough to develop the disease, the researchers say.