Cigarettes, cigars, chewing tobacco, snuff and pipe tobacco consist of dried tobacco leaves with other ingredients added to enhance flavor. More than 4,000 compounds are in tobacco and tobacco smoke, including 60 known cancer-causing agents.

Found naturally in all forms of tobacco, nicotine is a powerful drug that has addictive properties similar to heroin or cocaine. Inhaled with cigarette smoke, it is absorbed very quickly deep into the lungs. From smokeless tobacco (chewing tobacco or snuff), nicotine moves throughout the body via the nose and mouth. The drug can travel through the placenta from a pregnant woman to the fetus, and is also found in the breast milk of nursing mothers. Nicotine affects the heart, blood vessels, hormones, metabolism and the brain.

Nicotine gives smokers a pleasurable feeling and makes them want to smoke more. At the same time, it interrupts information traveling between nerve cells and acts as a depressant.

Over time the nervous system begins to tolerate the presence of nicotine. Smokers tend to increase the habit, sending more nicotine to the blood. Ultimately, nicotine reaches a certain level in the body and the individual smokes as many cigarettes a day as it takes to maintain that level.

Continue article to learn about the health risks of smoking.