Actually... 

It is both naïve and inaccurate to argue that drug use does not exist inside jails and prisons. Drugs are routinely brought into correctional facilities by visitors and corrupt guards¹ and one study reports that close to 60% of individuals who used hard drugs such as heroin and cocaine in the months prior to imprisonment continued to use the drug while serving their sentence ². The same study noted that nearly 70% of incarcerated heroin users still considered themselves to be dependent on the drug. Even if inmates were unable to access drugs while incarcerated, individuals being released from institutions are often returning to the same conditions from which they came, and the stigma attached to being an ex-con – including the difficulty in finding work – can be a factor that contributes to relapse.