Background: Pakistani jails face a rapidly expanding yet inadequately researched substance abuse crisis which targets inmates who previously used drugs before their incarceration and those inmates who start using drugs during their imprisonment. Prison populations show diversity in substance use because some undergo incarceration with pre-existing disorders from drug-related offenses while formerly abstinent inmates begin using drugs in response to mental stress along with peer pressure and widespread drug supply within their cells. The parameters behind substance abuse development in prison conditions receive analysis in this research alongside the systems which maintain addiction among prisoners. Methods: The research team conducted qualitative phenomenological interviews using a basic with 30 prisoners from three secure prisons across Pakistan. Our research included participants who used drugs before entering prison and prisoners who developed substance use habits during incarceration. Thematic analysis was done to find major themes and subthemes. Results: Three themes emerged. The first theme was, “Diverging Paths to Substance Use,” This theme reflects the factors behind preincarnation drug use and incarceration induced substance abuse. It also addresses disrupted or sustained drug addiction. The second theme “Drugs Behind Bars” narrates availability and access disparities, mental health and coping strategies and power structures and coercion. Third theme was “The Rehabilitation Paradox,”. This theme identified the failure of prison-based treatment and struggles for reintegration after being released. Conclusion: Our findings show prisons require specialized rehabilitation programs for their different types of drug users. Effective treatment during and after prison helps end drug trade inside and lowers the chances of former inmates returning to crime.