There's a reason why it's called an addiction; it's very difficult to quit smoking once you've picked up the habit. But no matter how long you've been a smoker, it's never too late to stop.
You already know that smoking can give you lung cancer (throat cancer, mouth cancer, pancreatic cancer -- you get the picture), but did you know that it can increase your risk of heart disease?
Smokers have a higher incidence of atherosclerosis (buildup of fat in the arteries), and they generally have lower HDL (the good kind of cholesterol). Smoking can increase your blood pressure and your risk of blood clots. And, if you're a heavy smoker, you've probably noticed that you get very out of breath if you try to do sustained aerobic activity -- something that's necessary for good heart health.
Even if you're not a smoker, secondhand smoke can also increase your risk of heart disease, so it's best to stay out of smoky places.