Drugs are generally prescribed to produce therapeutic effects. Certain drugs are capable of altering an individual's mood and perception, often generating different kinds of pleasurable experiences. This may lead to drug dependence—a state of compulsive and persistent drug use with possible adverse effects. Drug dependence can be categorized as psychological dependence or physical dependence. In psychological dependence, the drug-induced rewarding experience can strongly influence the person's desire to repeat the high and relapse. In physical dependence, a person experiences withdrawal or abstinence symptoms if the drug is abruptly discontinued or an antagonist is administered. This disturbs the homeostatic set point. Nicotine, an active ingredient in tobacco, has psychological and physical dependence. A person trying to quit smoking may experience withdrawal symptoms. If these symptoms are overcome, the person might be able to quit smoking temporarily. However, with time, depression and psychological cravings develop, which leads to relapse.