Background: Attention deficit hyperactivity disorder (ADHD) and behavioral addictions (BAs) are highly comorbid but little is known about the effect of anti-ADHD medications on behavioral addiction symptoms. Thus, the aim of this naturalistic prospective study was to investigate the long-term changes on BAs symptoms among methylphenidate-treated adults with a primary diagnosis of ADHD. Methods: 37 consecutive adult ADHD outpatients completed a baseline and follow-up assessment of ADHD, mood and BAs symptoms (internet, shopping, food, sex addictions and gambling disorder) after one year of methylphenidate (flexible dose) treatment. Results: Internet addiction test scores pre-treatment were significantly higher than post-treatment scores (p < 0.001). The same trend was seen for the shopping addiction (p 5 0.022), food addiction scores (p 5 0.039) and sex addiction scores (p 5 0.047). Gambling disorder scores did not differ pre and post treatment since none of the included patients reported significant gambling symptoms at baseline. The rate of ADHD patients with at least one comorbid BA was reduced after methylphenidate treatment (51.4% vs 35.1%). The correlation analyses showed a moderate positive correlation between the changes in sluggish cognitive tempo symptoms, cognitive impulsivity, mood and anxiety symptoms and changes in internet addiction symptoms. Conclusions: This is the first study showing that after one-year of treatment with methylphenidate, adult ADHD patients show a significant reduction on internet, food, shopping and sex addiction symptoms. Further controlled studies with larger samples should replicate these preliminary results and elucidate the role of methylphenidate and other moderator factors (such as concomitant psychological treatments or lifestyle habits changes) on BAs improvements.  Kulcsszavak: methylphenidate, ADHD, internet addiction, sex addiction, shopping addiction, food addiction