The term, substance abuse, refers primarily to the recreational use of drugs and is not considered a disorder unless it impairs social or occupational functioning. Drug dependence, a disorder ordinarily resulting from the repeated use of drugs, is manifested in a strong desire to continue taking the drug, either for the pleasant sensations that might result of to escape feelings of withdrawal. The APA distinguishes between physiological dependence, commonly called addiction, in which the desire to continue taking the drug is at least partly organically based, and psychological dependence, sometimes called habituation, in which the desire to continue taking the drug has to do mainly with its psychological rather than its physiological effects. Drug tolerance refers to changes that occur in the user so that with the passage of time more and more of the drug is required to produce the desired effect. The most commonly abused drugs are narcotics (e.g., opium, morphine, heroin, codeine, methadone), sedatives (e.g., barbiturates, tranquilizers, alcohol), stimulants (e.g., cocaine, crack, amphetamines), hallucinogens (e.g., LSD, PCP, mescaline, psiolcybin, marijuana), inhalants (e.g., glue, paint thinner, aerosol sprays, solvents), unclassified (e.g., nicotine), and "designer" drugs (any of a combination of chemicals and drugs, often manufactured by amateur chemists).