But  doctor, can't I have just 1 cigarette per day?

Almost everybody realises these days that cigarette smoking increases the risk of heart disease, stroke and cancer, and do their best to stop. Nicotine is probably one of the most addictive of all substances in some people, and many find it almost impossible to stop, and cut down to one or 2 cigarettes per day, in the belief that this dramatically reduces the risk. If you smoke 20 cigarettes a day and cut down to one, the assumption is that you reduce your risk 20 times.

Unfortunately a recent paper in the British Medical Journal is bad news for these people. They reviewed over 200 studies between 1946 and 2015, involving over 3 million men and 2 1/2 million women, and looked at the risk of having a heart attack or stroke if they smoke 20 cigarettes a day, or just one.
In men compared to non-smokers, the risk of having a heart attack was increased by 150% with one cigarette, and 200% with 20.  In women it was 220% and 390%.

So it appears that much of the damage is done by just one single cigarette, and complete abstinence is the only answer if we want to reduce the risk of heart attacks.
The incidence of stroke is similar.

To quote the paper: "smokers who cut down the number of cigarettes believe they can benefit from large reductions in the risk of cancer and cardiovascular disease. The study shows clearly that there is no safe level of smoking for cardiovascular disease, and light smokers cannot assume that continuing to smoke does not lead to harm. Smokers need to quit completely rather than cut down."