There is good evidence that effects of smoking on dementia are very powerful. Not everyone who smokes can get dementia, but avoiding smoking is thought to reduce the risk down to the level of non-smokers.
Any of the reasons for this include the fact that the two most common types of dementia, Alzheimer's disease, and vascular dementia, have been related to issues with the vascular system (your heart and blood vessels). It is understood that smoking raises the risk of vascular complications, including strokes or minor brain bleeding, which are both risk factors for dementia.
Also, toxins in cigarette smoke increase oxidative stress and inflammation, both associated with the development of Alzheimer's disease.
Research that examines the relationship between smoking and dementia specifically is complicated for several reasons. This includes the following:
There are several different chemicals and pollutants in cigarette smoke, so it is not clear which would cause harm. There is evidence that (nicotine) decreases the risk of dementia.
As smoking is the leading cause of premature death, many smokers are likely to die before they reach the age at which dementia develops. It is also likely that smokers who live to old age will have specific characteristics, such as genetics, which means that they are not indicative of the entire population. Studies involving these people could now offer an accurate picture of the risk to the population as a whole.
Any demographic studies measure people and their smoking habits at one point in time. These studies do not provide fully reliable results, since they require people to remember how much they smoked, and people may not be able to do so precisely, particularly if they already have symptoms of dementia. A more reliable way to collect this data is to track a large number of people over a long period and document their smoking habits as part of the research.
Many of the lifestyle risk factors for dementia are difficult to separate. For example, people who smoke a lot are more likely to drink alcohol, another known risk factor for dementia.
There is evidence that certain studies that have identified potentially beneficial effects of tobacco have been affected by the tobacco industry.
As some studies have been conducted, it is best to combine them to obtain the most consistent results. This is achieved by a method called a systematic analysis. The 2014 World Alzheimer's Report examines the conclusions of seven systematic studies and also carries out its own. A total of 14 studies were included in the study and the researchers found that there was a statistically significant rise in the risk of dementia in current smokers relative to those who have never smoked.
Smoking was also one of the nine modifiable risk factors outlined by the Lancet Commission on Dementia Risk in 2017.
Overall, systematic reviews have concluded that smoking increases the risk of developing dementia by about 30-50 percent. You will see our online Dementia Risk tool to understand what this means (scroll to the third screen to see the figures for smoking).
Does the amount of smoke that you smoke affect the risk of dementia?
There were only four studies in the World Alzheimer's Review that looked at the number of cigarettes smoked and the risk of dementia. Two have shown a link between higher tobacco use and a higher risk of dementia, although there is currently not enough data to know for sure if this is the case.
Will smoking cessation minimize the risk of dementia?
Data shows that avoiding smoking decreases the risk of dementia. This is analogous to other smoking results where smoking cessation contributes to a decrease in the risk of cardiovascular disease or cancer.
Can nicotine minimize the risk of dementia?
There is some evidence that exposure to nicotine, which is one of the components of cigarette smoke, can reduce the risk of dementia. Any of this study has been financed by tobacco companies, which does not invalidate it, but complicates its assessment. Although such reports may be useful in indicating potential avenues for research into the design of drugs, any benefit will likely be offset by the damage caused by the many other harmful components of cigarette smoke.
Can exposure of 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 side effects. It will also be predicted to raise dementia. Several studies have indicated that this could be the case (summarized in this report), but there are still no systematic reviews available.
Smoking is the leading cause of preventable death due to heart disease, stroke, and lung cancer. It is a risk factor for lung disease, diabetes, rheumatoid arthritis, and poor immune function, as well as adversely affecting fertility and maternal health. There are therefore several general health explanations for not smoking.
Some of the known consequences of smoking are known causes of dementia, and there is evidence that current smokers are more likely to develop dementia. Some researchers report that 14 percent of cases of dementia worldwide could be caused by smoking.
On a positive note, though, smoking cessation decreases this risk, so it is never too late to make healthy lifestyle choices. It's also likely to benefit if second-hand smoke is prevented.