Children with ADHD are 'a group at risk' as far as their further emotional and social development and educational possibilities are concerned, and the consequences of the lack of an appropriate therapy appears to be serious. Some of these children do not respond to prevailing therapy methods. It is ...
The effects of magnesium physiological supplementation on hyperactivity in children with attention deficit hyperactivity disorder (ADHD). Positive response to magnesium oral loading test - PubMed
It's sold as a nutritional supplement. Western countries at least don't seem to regulate it, it's pretty easily available, and since it's a supplement you probably don't need a prescription.