No doubt that, even as we speak, worried pundits are writing of how this is a worrying Orwellian scenario and yadda yadda. But what's really going on?
The research is from Sweden and published in the New England Journal of Medicine: Medication for Attention Deficit–Hyperactivity Disorder and Criminality. The first thing to note is that the study is not about giving medication in order to prevent crime; it was purely looking at what happened to people given ADHD treatment for their ADHD.
In a nutshell, the authors found that people diagnosed with ADHD were about 10% less likely to be convicted of a crime during periods when they were on medication for the disorder. This was true of both men and women, and the effect was greater for the more serious offences.
It was a huge study with over 25,000 ADHD patients and the data comprise pretty much everyone in Sweden over the relevant period so in that respect it's a very good study - although speaking of Orwellian, these studies are only possible because of the Scandinavian tendency to make national registers of everything.
Now the big criticism here is that it's just a correlation, it doesn't prove that the meds were what prevented crime. It might be that ADHD meds have no effect on crime, but that people are less likely to commit crimes at periods when they have their lives sorted out (when they're 'on the rails'), one marker of which is that they're seeking treatment for their ADHD.
However, the authors found that periods of use of SSRI antidepressants were not associated with changes in conviction rates. This is quite good evidence against the 'on the rails' critique, assuming that being prescribed SSRIs is as much a marker of being on the rails as being prescribed Ritalin is.
So, in my view, this is pretty good work, as good as any observational non-randomized study. However, remember: this is just about treating ADHD. Not drugging criminals to stop crime.
Lichtenstein P, Halldner L, Zetterqvist J, Sjölander A, Serlachius E, Fazel S, Långström N, and Larsson H (2012). Medication for attention deficit-hyperactivity disorder and criminality. The New England journal of medicine, 367 (21), 2006-14 PMID: 23171097