The Autism Research Institute has published the results of their survey comparing drugs, supplements and diets used in treating autism. Antifungal and seizure medications had mostly positive results, but ADHD medications yielded more negative outcomes than positive ones. Most supplements showed a high percentage of improvement and only small numbers of children whose behavior worsened.
The fewest negative effects, though, were reported for the diets. Both the Feingold Diet and Candida Diet helped 56%, the Gluten-Free/Casein-Free Diet helped 66% and the Specific Carbohydrate Diet helped 69%.*
Interestingly, a Gluten-Free/Casein-Free Diet or the Specific Carbohydrate Diet is also often used to address ADHD. The Feingold Diet’s avoidance of synthetic additives also probably ameliorates hyperactivity. See American Academy of Pediatricians on Food Additives; my family, unbeknownst to them, has also enjoyed recipes from The Kid-Friendly ADHD and Autism Cookbook: The Ultimate Guide to the Gluten-Free, Casein-Free Diet.
*First two paragraphs adapted from Pure Facts, the newsletter of the Feingold Association of the United States.