A drug is a substance that produces a psychoactive, chemical or medicinal effect on the user. The psychoactive effect of mood-altering drugs is modulated by the user's perception of the risks of drug use, his or her ability to control drug use and the demographic, socioeconomic and cultural context. The ability to control drug use may vary along a continuum from compulsive use at one end to controlled use at the other. The "drug problem" has been socially constructed, and the presence of a moral panic has led to public support for the prohibitionist approach. The legalization approach has severely attacked the dominant prohibitionist approach but has failed to gain much support in society because of its extreme libertarian views. The harm reduction approach, which is based on public health principles, avoids the extremes of value-loaded judgements on drug use and focuses on the reduction of drug-related harm through pragmatic and low-threshold programs. This approach is likely to be important in tackling the drug problem in the 21st century.