A treatment for autism in which an effective amount of lactulose is administered in order to bind excess ammonia in the gastrointestinal tract, the bloodstream, and the nervous system in order to prevent or reverse ammonia poisoning caused by the administration of certain antibiotics. Lactulose molecules in the colon are fermented by certain bacteria. The fermentation process lowers the colonic pH, and ammonia, in the form of ammonium ions, is used by the bacteria for amino acid and protein synthesis. This lowers the serum ammonia levels and reduces neurotoxicity.