There is strong evidence that smoking can increase your risk of developing dementia. Not everyone who smokes will get dementia, but stopping smoking is thought to reduce your risk back down to the level of non-smokers.
Some reasons behind this include the fact that the two most common forms of dementia, Alzheimer's disease and vascular dementia, have both been linked to problems with the vascular system (your heart and blood vessels). It is known that smoking increases the risk of vascular problems, including via strokes or smaller bleeds in the brain, which are also risk factors for dementia.
In addition, toxins in cigarette smoke increase oxidative stress and inflammation, which have both been linked to developing of Alzheimer's disease.
What does the research say?
Research that studies the relationship between smoking and dementia directly is complex for several reasons. These include:
- There are many different chemicals and toxins in cigarette smoke so it is unclear which ones would be causing damage. There is some evidence that one (nicotine) actually reduces the risk of dementia.
- As smoking is a leading cause of premature death, many smokers are likely to die before they reach the age at which dementia will develop. It is possible therefore that smokers who survive to old age may have particular traits, for example genetics, that mean they are not representative of the whole population. Studies involving these people may now give an accurate idea of the risk to the population as a whole.
- Some population studies compare people and their smoking habits at one particular point in time. These studies may not produce completely accurate data, because they require people to remember how much they smoked, and people may not be able to this accurately, especially if they already have symptoms of dementia. A more accurate way to gather this data is to follow a large amount of people over a long period of time, and record their smoking habits as part of the study.
- Many of the lifestyle risk factors for dementia are hard to separate out. For example, people who smoke a lot are more likely to consume alcohol, another known risk factor for dementia.
- There is evidence that some studies that found potential protective effects of tobacco were influenced by the tobacco industry.
As there have been many studies, it is better to combine these in order to obtain the most consistent findings. This is done using a process called a systematic review. The World Alzheimer's Report 2014 examines the findings from seven systematic reviews and also carries out its own. A total of 14 studies were included in the review, and the researchers found that there was a statistically significant increased risk of dementia in current smokers compared to people who have never smoked.
Smoking was also one of the nine modifiable risk factors highlighted in the 2017 Lancet Commission on dementia risk.
Overall, systematic reviews have estimated that there smoking confers between a 30-50% increased risk of developing dementia. To understand what this actually means, you can see our online Dementia Risk tool (scroll to the third screen to see the figures for smoking).
Does the amount you smoke affect risk of dementia?
There were only four studies in the World Alzheimer's Report review that looked at the number of cigarettes smoked and dementia risk. Two did show a relationship between higher tobacco consumption and a higher risk of dementia, but it currently there is not enough evidence to know for sure whether this is the case.
Can stopping smoking reduce dementia risk?
The evidence is that stopping smoking reduces your risk of dementia. This is similar to other findings with smoking, where stopping smoking leads to a reduction in the risk of cardiovascular disease or of cancer.
Can nicotine reduce dementia risk?
There is some evidence that exposure to nicotine, which is one of the components of cigarette smoke, can actually reduce the risk of dementia. Some of this work was funded by tobacco companies, which doesn't invalidate it but does complicate its evaluation. While such reports may be useful in indicating possible research directions for drug design, it is likely that any benefits would be outweighed by the harm caused by the many other toxic components in cigarette smoke.
Can exposure by non-smokers to tobacco smoke (second-hand smoking) cause dementia?
Second-hand smoke has been shown to increase the risk of cancer, cardiovascular disease, and other effects. It would therefore be expected to increase dementia too. Several studies have suggested that this might be the case (summarised in this report), although there are no systematic reviews available yet.
What should I take away from this research?
Smoking is the main cause of avoidable death, through heart disease, strokes and lung cancer. It is a risk factor for lung disease, diabetes, rheumatoid arthritis and poor immune function, and also adversely affects fertility and maternal health. There are therefore many general health reasons for not smoking.
Some of the known effects of smoking are known causes of dementia, and there is evidence that a current smoker is more likely to develop dementia. Some researchers estimate that 14% of dementia cases worldwide may be attributable to smoking.
However, on a positive note, stopping smoking reduces this risk, so it is never too late to adopt healthier lifestyle choices. It is also likely to help if second-hand smoke is avoided.