One drop of pure nicotine can be fatal to humans. It is a powerful stimulant to the brain and central nervous system that 'hits'' the brain within four seconds. Like the drug, alcohol, after it initially stimulates, it has a depressant effect. Your heart has to pump harder, thus increasing the chance of heart disease. Even after one stops smoking, carbon monoxide stays in the bloodstream for hours, depriving the body of oxygen until the oxygen level in your blood returns to normal.
Carbon monoxide is a waste product of cigarette smoking and also of gasoline engines.
Smoking is a contributing factor in cancer of the lungs, mouth, and esophagus. The effects of some medication taken by a person may be increased, decreased, or cancelled out by smoking. Cigarette smoking by pregnant women may cause harm to the fetus. Cigarette smoke pollutes the air in enclosed places, which also affects the nonsmokers present. Chewing tobacco may lead to cancer of the mouth and to an addiction because of the nicotine that is absorbed through the mouth's lining.