Nicotine is an alkaline chemical which acts as a stimulant, raising blood pressure, increasing the heart rate and increasing the uptake of sugars. It also acts as an appetite suppressant. It is the primary active chemical in tobacco products. It is readily absorbed into the body through the tissues of the mouth, the mucous membranes of the nose and lungs, the skin and the digestive tract.
Although the typical dose of nicotine in a cigarette will only provide mild stimulation (unlike cocaine or amphetamines), it is very addictive, on the same level as opiates. Withdrawal symptoms can be severe, such as nervousness, need for oral stimulation, and increase in appetite. The symptoms of withdrawal can last for a very long period of time as nicotine has a half-life in the body and is only removed very slowly.
Cigarettes allow a very controlled dose of nicotine to be absorbed into the body. Unfortunately, substitutes like nicotine gum and the nicotine patch are unable to match the increased release rate of cigarettes, which essentially allow the increase of the dosage at will. As such, although substitutes can help with withdrawal symptoms, they do not provide the same stimulation as tobacco products do.
Ironically, nicotine is one of the least lethal products of tobacco products (although it is highly poisonous in high doses). The other ingredients of tobacco are the ones that cause damage to the lungs. As such, substitute products are very safe to use.
Nicotine in higher doses can also be used as an insecticide.