"Imprisonment forces some drug users to stop using drugs, and some will see this as an opportunity to improve their lives. For others, however, prison may be a setting for initiation into drug use or for switching from one drug to another, often due to lack of availability of the preferred drug inside prison (Fazel et al., 2006; Stöver and Weilandt, 2007) and other possible reasons (e.g. use of substances for which avoiding control measures is easier). Sometimes, this change leads to more harmful patterns of drug use (Niveau and Ritter, 2008). For example, a Belgian study carried out in 2008 found that more than one-third of drug-using prisoners had started to use an additional drug during detention, one that they were not using before entering prison, with heroin being the drug most frequently mentioned (Todts et al., 2008)."


European Monitoring Centre for Drugs and Drug Addiction, "Prisons and drugs in Europe: the problem and responses" (Luxembourg: Publications Office of the European Union, November 2012), Catalog No. TDSI12002ENC, doi: 10.2810/73390, p. 10.