Mark Twain said, "Quitting smoking is easy. I've done it a thousand times." Why is it so hard to quit smoking? Nicotine is one reason it is so difficult to quit smoking. Nicotine is a drug found naturally in tobacco. It is highly addictive, as addictive as heroin and cocaine. Over time, the body becomes physically and psychologically dependent on nicotine.
Nicotine provides an almost immediate "kick" due to a discharge of epinephrine from the adrenal cortex. This results in a release of excess glucose which is followed by depression and fatigue, leading smokers to seek more nicotine.
Nicotine produces pleasurable feelings that make the smoker want to smoke more. As the nervous system adapts to nicotine, smokers tend to increase the number of cigarettes they smoke, and subsequently the amount of nicotine in their blood. After a while, the smoker develops a tolerance to the drug, which leads to an increase in smoking over time.
When you inhale cigarette smoke, nicotine and other harmful chemicals such as carbon monoxide (CO), ammonia, formaldehyde and hydrogen cyanide are carried deep into your lungs, where they are absorbed quickly into your bloodstream and carried throughout your body. The chemicals from cigarette smoke affect many parts of your body, including your heart and blood vessels, your hormonal system, your metabolism, your brain and during pregnancy, your fetus.
Carbon monoxide (CO) is a colorless, odorless gas that reduces the blood's ability to carry oxygen. Ammonia is a colorless gas that can be very irritating to the skin, eyes, nose, throat, and lungs. Ammonia is used to boost the impact of nicotine, also known as freebasing. The end result is an enhanced effect of the nicotine. Formaldehyde is classified as a human carcinogen (cancer-causing substance) by the International Agency for Research on Cancer, and as a probable human carcinogen by the U.S. Environmental Protection Agency(EPA). Hydrogen cyanide is very toxic.
Cigarettes contain some 599 possible additives and over 4000 chemical compounds are created by burning a cigarette, many of which are toxic and/or carcinogenic. Forty-three known carcinogens are in mainstream smoke, sidestream smoke, or both.
According the American Heart Association, cigarette smoking is the most important preventable cause of premature death in the United States. It accounts for more than 440,000 annual deaths. Cigarette smokers have a higher risk of developing a number of chronic disorders. These include fatty buildups in arteries, several types of cancer and chronic obstructive pulmonary disease (lung problems). Atherosclerosis (buildup of fatty substances in the arteries) is a chief contributor to the high number of deaths from smoking.
Many studies detail the evidence that cigarette smoking is a major cause of coronary heart disease, which leads to heart attack. Cigarette and tobacco smoke, high blood cholesterol, high blood pressure, physical inactivity, obesity and diabetes are the six major independent risk factors for coronary heart disease that you can modify or control. Cigarette smoking is so widespread and significant as a risk factor that the Surgeon General has called it "the leading preventable cause of disease and deaths in the United States."
Source: American Cancer Society and Quitsmoking.about.com
American Heart Association