Smokers are more than twice as likely as non-smokers to develop dementia and Alzheimer's disease, according to a new study published in the international medical journal, The Lancet. The study of nearly 7000 people aged over 55 and living in Rotterdam, found smoking was a strong risk factor for both men and women. The study, by scientists from a number of Dutch universities,found no association between smoking and Alzheimer's in people who carried an uncommon gene, known as APOE4. However, Melbourne scientists say the gene is a genetic riskfactor for Alzheimer's.