Hi there, you might want to look at this article: https://www.ncbi.nlm.nih.gov/pubmed/20034331.
It says near the end of the article that “A study of ADHD treatment with zinc sulfate as a supplement to methylphenidate showed beneficial effects of zinc supplementation in the treatment of children with ADHD. The dose of zinc sulfate used was 55 mg/day, which is equivalent to 15 mg zinc.”