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Smoking is bad for your health. It's harmful because there are so many things in tobacco smoke that can harm your body. The most common diseases caused by smoking are lung cancer, heart disease and stroke.

There are more than 4,000 chemicals in tobacco smoke. Many of these are poisons. At least 43 of these chemicals cause cancer. Every time you breathe in tobacco smoke, you breathe in poisons that harm your body.¹ Smoke also contains tar and harmful gases, such as carbon monoxide.

• The tar and chemicals in tobacco smoke cause your lungs to make a thick fluid called mucus. It clogs your lungs and can cause a "smoker's cough."
• There are tiny hairs that help "sweep" germs and other things out of your lungs. Those hairs can't move easily when you have mucus and tar in your lungs. So you're more likely to get an infection in your lungs.
• The chemicals in smoke make the walls of your airways swell and get thicker. This makes the airways narrower, so you get less air when you take a breath.
• The walls of your lungs have many tiny bags (or sacs) that hold air. Cigarette smoke damages those sacs. When that happens, less oxygen gets into your blood, and you get out of breath and get tired more easily. Eventually, you can get a lung disease called chronic obstructive pulmonary disease (COPD). If you have this, it gets harder and harder for you to breathe.
• The chemicals in smoke can lead to lung cancer. The chemicals damage cells in your lungs, and the damaged cells can then become cancer cells. The more you smoke, the greater your chance of getting lung cancer. If you smoke cigars or a pipe, you also have a higher chance of getting lung cancer than people who don't smoke.² To learn more, see our articles on lung cancer.
• Carbon monoxide from cigarette smoke gets into your blood. It reduces the amount of oxygen your blood can carry. This means your heart has to work harder to get enough oxygen to all the cells in your body. The extra work puts a strain on your heart, which over many years can lead to heart problems.
• Some particles in smoke seem to help "bad" cholesterol in your blood stick to your blood vessels. When cholesterol builds up in the blood vessels, it makes them narrower and increases the chances that you'll have a heart attack or stroke.

We can't say for sure what will happen to you as an individual if you keep smoking. Everyone is different. This is what we know from studies of people who smoked.

• About half of all smokers die of a disease that's caused by smoking. The big killers are lung cancer, heart disease and stroke.³
• On average, the life of someone who smokes is 13 or 14 years shorter than the life of someone who doesn't smoke.
• If you smoke, you're more likely to get cancer in your lungs, intestine, throat, pancreas, kidneys, bladder or cervix.
• Smoking is the most important cause of chronic obstructive pulmonary disease (COPD).
• Cutting down on the number of cigarettes you smoke doesn't improve your health. You have to stop completely to benefit.³ But cutting down may be a good first step toward quitting completely, if you find it too hard to stop right away.⁴

If you smoke when you're pregnant, you can damage your baby's health. The chemicals in smoke keep babies from getting all the oxygen and nutrients they need to grow. Here's what we know from the research.⁵

• Your baby is likely to weigh less than it would have if you hadn't smoked. Infants who don't weigh enough are likely to have breathing problems.
• Your baby may be born early and need special care.
• As your baby grows, he or she is more likely to get asthma.
• As many as 1 in 10 infant deaths are due to the mother smoking while she was pregnant.

Breathing in smoke from other people's cigarettes can cause health problems in people who don't smoke. Second-hand smoke can:⁶

• Cause lung cancer and heart problems (such as heart attacks and stroke) in people who don't smoke
• Make children's asthma worse
• Cause pneumonia, ear infections, bronchitis, coughing and wheezing in young children
• Lead to lung infections in children. Up to 15,000 children need hospital treatment each year because of breathing problems.

Almost as soon as you stop smoking, your body starts to clear itself of the poisonous chemicals found in smoke.⁷ Here's what happens.

• Within hours. The amount of carbon monoxide in your blood will return to normal within 48 hours. (Carbon monoxide keeps oxygen from getting into your blood and makes it harder for you to breathe.)
• Within days. Your sense of taste and smell will get better.
• In 1 to 2 months. If you have a long-term cough, symptoms like coughing, producing phlegm and wheezing will improve.
• In 1 year. If you have COPD (a condition which makes it hard for you to breathe), your breathing will get easier.
• In 5 years. For women, your risk of cervical cancer drops to the same as someone who's never smoked.
• In 10 to 15 years. Your risk of coronary heart disease (which causes heart attacks) is about the same as someone who's never smoked.
• In 10 to 20 years. Your risk of lung cancer falls to 70 percent of what if would be if you kept smoking.

The sooner you stop smoking, the better. If you stop smoking before the age of 35, you avoid 90 percent of the health problems linked with smoking.⁸ But even if you quit after the age of 50, you reduce your chances of dying from a disease linked to smoking.

Stopping smoking increases your chance of living longer. On average, if you give up smoking:⁹

• At age 30, you'll gain an extra 10 years of life
• At age 40, you'll gain an extra 9 years of life
• At age 50, you'll gain an extra 6 years of life
• At age 60, you'll gain an extra 3 years of life.

See Other ways quitting helps for more information.

Giving up smoking also has its problems. But most of them are temporary, and the improvement in your health will soon outweigh these short-term problems.

• You may get depressed, feel anxious, or find it harder to concentrate.¹⁰
• There's some evidence that smokers who quit get more sore throats, coughs, sneezing and other cold symptoms than those who don't.¹¹ These symptoms seem to last for just two weeks after quitting and then they go away.
• For many people the biggest drawback to quitting smoking is putting on weight. The average is about 8 pounds to 13 pounds. ¹²

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.