"In the most common English usage decriminalisation is the elimination of a conduct or activity from the sphere of criminal law, while depenalisation is simply the relaxation of the penal sanction provided for by law. The term decriminalisation is most commonly used in reference to offences related to drug consumption and usually manifested by the imposition of sanctions of a different kind (administrative) or the abolition of all sanctions; other (non-criminal) laws can then regulate the conduct or activity that has been decriminalised. Depenalisation can refer to consumption-related offences (which may be dealt with through referral schemes or alternative sanctions for drug users) but also to small-scale trading, generally indicating elimination or reduction of custodial penalties, although the conduct or activity remains a criminal offence. Confusingly, in Spanish, depenalisation often refers to what in English is most often called decriminalisation. Legalisation is the removal from the sphere of criminal law of all drug-related offences: use, possession, cultivation, production, trading, and so on. Regulation refers to a strictly controlled legal market, in which administrative rather than criminal law regulates production, distribution and price (by taxation); and limits availability and access, using models developed for pharmaceutical drugs, alcohol and tobacco."

Source

Jelsma, Martin, "The Development of International Drug Control: Lessons Learned and Strategic Challenges for the Future," Global Commission on Drug Policies (Rio de Janeiro, Brazil: January 2011), p. 9.
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