Anyone who smokes is at risk of becoming addicted to nicotine. Tobacco smoking is a behavior that often becomes established during adolescence. About 90 percent of smokers begin smoking before the age of 21, and most start before age 18.

Among adolescents, nicotine addiction can begin within days or weeks of smoking their first cigarette, even among occasional users. Exposure to tobacco in movies or advertisements, particularly for young people, can become strong reinforcement for a decision to start smoking or keep smoking. Antismoking ads are on the rise, however, and most high-school students now see an equal number of smoking and antismoking ads. On the other hand, the amount of smoking seen in movies has increased.

You may have tried to stop smoking because you've experienced health problems related to smoking or because you're concerned about potential health problems. If you've tried to stop, but have discovered you can't, talk to your doctor about which approaches to take.

When you inhale tobacco smoke, you're ingesting a chemical parade that will march through most of your body's vital organs. The negative health effects throughout your body are numerous, including:

• Lungs. Smoking is the cause of 87 percent of all lung cancer. Smoking is also the primary cause of other lung problems, such as emphysema and chronic bronchitis.

• Heart and circulatory system. Smoking is associated with an increased risk of cardiovascular disease. The reason for this is unclear, but researchers suspect that nicotine triggers your adrenal glands to produce hormones that stress your heart by increasing blood pressure and heart rate. Carbon monoxide in tobacco smoke steals oxygen from the heart and other vital organs. Smoking may also constrict your blood vessels, placing you at greater risk of a heart attack or stroke.

• Cancer. Smoking is associated with cancer of the esophagus, larynx, pharynx, mouth, bladder, pancreas, kidney and cervix.

• Appearance. The chemicals in tobacco smoke can dry and irritate your skin and promote wrinkles. Smoking also yellows your teeth, fingers and fingernails.

• Fertility. Smoking increases the risk of infertility and miscarriage in women, and the risk of impotence and infertility in men.

• Sudden infant death syndrome (SIDS). Mothers who smoke while pregnant greatly increase the risk of SIDS in their newborns. Exposure to secondhand smoke in a newborn also increases the risk of SIDS.

• Senses. Smoking deadens your senses of taste and smell, so food isn't as appetizing as it once was.