Cigarette smoke contains more than 4,000 different chemicals, with nicotine being the primary component. Nicotine is a naturally occurring colorless liquid that comes from the tobacco plant. It is considered an addictive drug, making cigarette smoking the most prevalent form of nicotine addiction in the United States.
Cigarette smoking is a cause of many preventable diseases worldwide. The effect nicotine has on the brain and the body is complex. With each cigarette, a smoker takes in approximately 1 to 2 mg of nicotine. The smoker immediately experiences a "high" after exposure to nicotine, due to the drug's stimulation of the adrenal glands and resulting secretion of epinephrine, also known as adrenaline. The rush of adrenaline stimulates the body and causes an increase in blood pressure, respiration, and heart rate.
Nicotine also causes the release of dopamine in the brain; dopamine is a chemical responsible for sensations of pleasure. The release of dopamine causes the smoker to experience a sense of euphoria following nicotine exposure, similar to other psychoactive drugs. The feelings of a "high" and of euphoria enhance nicotine's addictive effect.
Repeated exposure to nicotine results in the development of tolerance, the condition in which higher doses of a drug are required to produce the same stimulating effects. When an adequate level of nicotine is not maintained, the smoker will experience symptoms of nicotine withdrawal.
These symptoms include decreased heart rate, difficulty concentrating, nervousness, headaches, increased appetite and weight gain, insomnia, irritability, and depression. While these symptoms will diminish over time, the adverse effects of nicotine withdrawal make it difficult for most people to quit smoking.