A study in the Journal of the American Medical Association (JAMA) has found that St. John’s Wort was not effective in treating childhood ADHD. Participants in the study either received St. John’s Wort or a placebo. There was no decrease in ADHD behaviors in either group.
I always ad a caveat that in the United States “nutraceuticals” or “natural supplements” are not regulated by the FDA. This means that if you are going to take a nutraceutical, you need to make sure the manufacturer has very strict quality control. And always tell your doctor when you are considering taking a nutraceutical, especially if you are already taking psychotropic medication. St. John’s Wort and some other nutraceuticals affect the central nervous system.
Here’s a link to an article on the ADHD/St. John’s Wort research: