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Nicotine (chemical formula C10H 14N2) is an alkaloid found primarily in leaves of the tobacco plant (Nicotiana tabacum). Many societies throughout the world have prized nicotine for its mood-altering properties: Although it is a stimulant, it can produce either relaxation or arousal, depending on the user's state (the relaxation appears occur on a muscular level). Users commonly burn the leaves and inhale the smoke; some, however, may chew the leaves, while others either "snuff" finely ground leaves into their noses or place them between their cheeks and gums.

Nicotine is so highly addictive that the American Psychiatric Association includes it in their diagnostic manual under substance dependence. Nicotine addiction is also very difficult to break—only 5% of those who attempt to quit smoking are successful on their first try, and only 3% can kick the habit for a whole year. Only 10% of smokers are not addicted. To relieve the physical and psychological symptoms of nicotine withdrawal—restlessness, anxiety, irritability, depression, difficulty in concentrating, and a craving for the drug—pharmaceutical companies now offer nicotine replacement systems such as the nicotine "patch" and gum. These systems deliver nicotine in a less addicting pattern that allows the dose to be gradually decreased and eventually eliminated. Even with nicotine replacement, however, successful quitting requires determination and is more successful when psychological support—like those offered in kicking other addictive substances—is given. Two drugs—clonidine, and the antidepressant bupropion (Wellbrutin)—have been approved by the Food and Drug Administration to help people quit.

Like most alkaloids, nicotine exerts its effects at receptors for chemicals that transmit nerve impulses. Specifically, nicotine acts at the nicotinic receptor class for the transmitter acetylcholine (the other class of acetylcholine receptor is the muscarinic, also named for a compound—a mushroom derivative—that triggers only receptors of that class). Outside the brain, nicotinic receptors are found primarily in the sympathetic nervous system, while muscarinic receptors are found in the parasympathetic nervous system. Thus, nicotine use triggers sympathetic nervous system effects throughout the body.

These effects largely account for nicotine's unfavorable impact on the user's health. People pay a great deal of attention to the danger of lung cancer, which results when smokers inhale cigarette smoke. While nicotine in itself is not carcinogenic, cigarettes and tobacco products contain more than 4,000 different chemicals, 60 of which are known carcinogens, and account for approximately one in every seven deaths in the United States, and one in three between the ages of 35 and 70—primarily due to cancers and cardiovascular diseases. Nicotine does, however, constrict small arteries, which raises the blood pressure and makes the heart work harder. It also makes the heart beat faster, yet, because it constricts the arteries supplying the heart muscle, the organ receives less blood. When buildups of fatty plaque have already narrowed heart arteries, this may be enough to trigger heart pain (angina) or heart attack. Also, elevated blood pressure greatly increases the risk of stroke. Nicotine causes circulatory problems, particularly affecting the hands and feet, and causes some men difficulty in obtaining an erection.

On the other hand, nicotine may have beneficial properties: for some users, it inhibits the appetite and slightly speeds up the body's metabolic rate, helping to keep weight down. Also, research has shown smokers appear to have a decreased risk of Parkinson disease.

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