Can i give my 6 months old 25 lb. Adult benadryl?

I took him to the vet for a chek up and stuff she said it was fine to give him benadryl for allergies we tried childrens benadryl but the effect seem to be too short and we had to give him constantly. A vet friend also told us that we could give him benadryl but children but he deals with small breed and hes pretty big so i belive he can take it just need some advice. Thanks