Random drug-testing of prisoners was introduced in 1995 in order to solve the many problems associated with prisoners taking drugs. Since cannabis can be detected in the body up to a month after having been smoked, prisoners are tempted to switch to heroin, which stays in the system for only 48 hours. As a result, since drug-testing was introduced, cannabis use has declined by a fifth whereas heroin use has doubled. Heroin is not only a much more damaging drug than cannabis, but it is also extremely addictive. There is evidence that heroin addiction encourages prisoners to intimidate others in order to pay for the drug.