Abstract: The invention relates to medicine, and more particularly to psychopharmacology, and is directed to an agent for treating and preventing autism spectrum disorders; said agent is glycine immobilized on detonation nanodiamond particles 2-10 nm in size, wherein the content of glycine is from 1 to 21±3 wt. %. The described agent improves the outcome of drug therapy and preventive treatment of autism in children and adults and expands the range of the effective and safe psychotropic medications.