Addictive drugs cause changes in the receptors to which they bind, making them less sensitive. This creates tolerance and addiction. Most addictive drugs work by altering levels of neurotransmitters in the brain's reward circuitry centered on the limbic areas. Other brain areas are also involved and each type of drug works in a slightly different way to produce its characteristic effects.

Nicotine

Nicotine, an element of tobacco smoke, is one of the most addictive substances that exists. When tobacco is smoked, most of the nicotine is paralyzed; a dose sufficient to cause mild somatic dependency and mild to strong psychological dependency remains. A study by Perrine concludes that nicotine's potential for psychological dependency exceeds all other studied drugs. Many smokers use nicotine compulsively. According to the American Heart Association, "Nicotine addiction has historically been one of the hardest addictions to break."

It can be difficult to quit smoking due to the withdrawal symptoms which include insomnia, irritability, anxiety, decreased heart rate, weight gain, and nicotine cravings. The relapse rate for quitters is high: about 60% relapse in three months. Also, nicotine users are sometimes reluctant to quit smoking because they do not see any short-term damages it may cause.

In chronic users, nicotine simply relieves the symptoms of nicotine withdrawal: confusion, restlessness, anxiety, insomnia, and dysphoria. Withdrawal symptoms in chronic users begin to appear approximately 30 minutes after every dose.

Medical research has determined that chronic tobacco smoking is a major contributing factor towards many health problems, particularly lung cancer, emphysema, and cardiovascular disease. Many governments now restrict or ban smoking in a variety of public venues over concerns of second-hand smoke, and ostensibly to discourage tobacco use in general. A cigarette contains over 4000 chemicals, over 60 of which are believed to increase the risk of cancer and other diseases in humans and animals.

Dopamine is one of the key brain chemicals actively involved in the desire to consume drugs. Research shows that by increasing the levels of dopamine within the reward circuits in the brain, nicotine acts as a chemical with intense addictive qualities. In many studies it has been shown to be more addictive than cocaine, and even heroin. Like other physically addictive drugs, nicotine causes pathological down-regulation of the production of dopamine and other stimulatory neurotransmitters as the brain attempts to compensate for artificial stimulation. When nicotine exposure ceases, the neural changes it creates in the brain and body are unpleasant. Also like other highly addictive drugs, nicotine is addictive to many animals besides humans.