The ADHD community, like many underserved populations, tends to be overrepresented in the prison population due to lack of accommodations and resources, along with stigma. There's a great deal of peer-reviewed research on the subject of ADHD and drug addiction. The overall gist is that people with ADHD are at higher risk for substance use disorders than their neurotypical counterparts due to several possible factors, including chronically low levels of dopamine and contextual factors like lack of resources and stigma.

Keep in mind that those numbers may be skewed based on how ADHD was diagnosed in the past. Only in the last few years have we started to see more traditionally "successful" people being diagnosed with ADHD, which is revealing that the percentage of the general population with ADHD is likely much higher than previously thought. ADHD was considered a "disease" that caused people to make trouble. That created a vicious cycle of stigma and punishment for those who had been diagnosed or expressed obvious symptoms. Meanwhile, lots of people with more subtle ADHD symptoms, stronger support systems, and greater privilege were never diagnosed at all. I'd take that 27% statistic with a big grain of salt, especially if it's coming from nootropics websites.