Leiby Kletzky, 8, had traces of cyclobenzaprine; an anti-psychotic drug, quetiapine; pain medication; and Tylenol in his blood, ABC reported.
Medical examiners determined the boy died July 13 of suffocation, and ruled his death a homicide.
Levi Aron allegedly told the police he smothered the child before dismembering him.
Kletzky was abducted while walking to his Brooklyn home from summer camp in Borough Park. His mother planned on meeting him half way, however, Kletzky got lost. He then allegedly asked Aron for directions and was not seen alive again.
Kletzky's body was found two days later. Dismembered body parts were found in Aron's freezer in his home in Kensington.
Charges are expected to pressed soon.