Prison and offending populations in Sweden and around the world are severely negatively selected in terms of their physical and mental health. This includes disorders like Attention Deficit and Hyperactivity Disorder (ADHD), which is not only over-represented but also trending up. Moreover, evidence that individuals commit fewer crimes while on ADHD medication has led to ADHD diagnoses/treatment being debated as a potential crime control policy. This paper uses detailed administrative data for Swedish prison admissions from 2009 to 2013 on in-prison healthcare combined with Swedish health (prescription, hospitalization, and mortality) and conviction registers to study the relationship between prison, in-prison ADHD treatment, and crime and mental health. First, using both DiD event study designs and spell-level OLS regressions with detailed longitudinal pre-prison health controls, we show that in-prison ADHD diagnoses significantly increase post-prison ADHD medication. This is driven by inmates with new ADHD diagnoses, and by those inmates who received ADHD medication while in prison.  Second, comparable reduced form analyses of the post-release effect of ADHD treatment find some short-term reductions in drug and alcohol convictions, though no overall effect on crime, and minimal evidence of an improvement in general mental health. These findings suggest that the sharp post-prison increase in ADHD medication does not explain the significant reduction in convictions and mental health problems observed after release from Swedish prisons. More generally, in-prison ADHD treatment does not appear to be a viable crime control policy. However, a silver lining of our analysis is the strong evidence that prison healthcare can bring high-risk populations into the public healthcare system. We demonstrate the external validity of this phenomenon with another in-prison health treatment.