Smoking and diabetes both increase the risk of heart disease in similar ways. This means that when combined they greatly increase your chance of developing a heart-related condition such as a heart attack or stroke.
High levels of glucose in your blood and smoking both damage the walls of your arteries (blood vessels) in a way that means fatty deposits (plaque) can build up much easier. This is known as atherosclerosis. As atherosclerosis occurs, your blood vessels narrow and therefore blood (and vital oxygen) flows through less easily.
When this happens to your coronary arteries (the arteries that supply the heart muscle with blood) you can have a heart attack or angina. When this happens to the arteries that take blood to your brain, you can have a stroke.