There is strong evidence that smoking can increase your risk of dementia. Not everyone who smokes will get dementia, but stopping smoking is thought to reduce your risk down to the level of non-smokers.

It is known that smoking increases the risk of vascular problems, including strokes or smaller bleeds in the brain, which are also risk factors for dementia. In addition, toxins in cigarette smoke cause inflammation and stress to cells, which have both been linked to the development of Alzheimer's disease.

What does the research say?

Research that studies the relationship between smoking and dementia 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.  
• As smoking is a leading cause of premature death, many smokers may not reach the age at which dementia normally develops.  It is possible therefore that smokers who survive to old age may have genes that protect them from developing some of the diseases that cause dementia. These cases are rare, and do not represent the whole population. 
• 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 do this accurately, especially if they already have symptoms of dementia. A more accurate way to gather this data is to follow a large number of people over a long time and record their smoking habits as part of the study. 
• Many of the lifestyle risk factors for dementia are hard to separate. 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 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 significantly increased risk of dementia in current smokers compared to people who have never smoked. 

Smoking was also one of the 12 modifiable risk factors highlighted in the 2020 Lancet Commission on dementia risk. Overall, systematic reviews have estimated that there smoking confers between a 30-50% increased risk of developing dementia.

Does the amount you smoke affect the risk of dementia?

There were only four studies in the World Alzheimer's Report that looked at the number of cigarettes smoked and dementia risk. Two showed a relationship between higher tobacco consumption and a higher risk of dementia, but 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 cancer. Smoking E-cigarettes is also likely to be far less damaging to health than cigarettes.

A Public Health England Report in 2018 stated that the risk of cancer from smoking e-cigarettes is 0.5% of the risk from normal cigarettes. Although this was not tested, the report also suggests that the risks of cardiovascular and lung disease are likely to be much reduced as well. If true, this would also have a positive impact on dementia risk.

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. Such reports may be useful in indicating possible research directions for drug design.

However, nicotine intake through smoking would not be beneficial as any positive effects would be outweighed by the significant harm caused by the many other toxic components in cigarette smoke.

Can passive exposure to tobacco smoke (second-hand smoking) cause dementia?

Second-hand smoke has been shown to increase the risk of cancer, cardiovascular disease, and other diseases. Studies have shown that it may also increase a person’s risk of developing dementia later in life. Research suggests that the more exposure to second-hand smoke a person has, the higher the dementia risk.

What should I take away from this research?

Smoking is the main cause of avoidable death, through heart disease, stroke, 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 the diseases that cause 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 where possible.