Research suggests that children with ADHD are more likely than their non-ADHD peers to drink alcohol and use drugs later in life, specifically as teenagers. A 2003 study published in the Journal of Abnormal Psychology found that more than a third of the ADHD participants (out of 140 teenagers who had been diagnosed as children) reported smoking marijuana, compared to just over a quarter of the non-ADHD control group. Nearly twice as many in the ADHD group reported being drunk more than once in the previous six months.
Although the exact connection remains unknown, experts believe ADHD and substance use are linked by a complex blend of environmental and genetic factors.
The role of age in substance use among people with ADHD is also an open question. Most studies have looked at teenagers, rather than adults in their 20s and beyond,so the ways in which ADHD affects substance use in different stages of life remain unclarified.
Phelps was diagnosed with ADHD in elementary school, Phelps's mother told the New York Times in 2008, when his teachers remarked on his lack of focus and inability to sit still. Between the ages of 9 and 11, he took Ritalin, a stimulant medication prescribed for ADHD. Ultimately, he asked to be taken off the medication because he felt stigmatized by going to the nurse's office every day.
Another explanation may be genetic. Both stimulant treatment and substance use involve dopamine and catecholamine, chemicals in the brain involved in pleasure, reward, and stress.