Benadryl is an antihistamine. It will have no effect on "mange" persay, if that is why you are using it.
There are two kinds of "mange" caused by two kinds of mite. You would need medicine from a vet to effectively treat either. The symptoms of mange - hairloss, itchiness, scabs, discolouration - may have other causes. These can include fleas, other parasites, bacteria, fungus, or allergies. You really need to seek veterinary attention to diagnose what the problem is before you start treatment.
There are two kinds of "mange" caused by two kinds of mite. You would need medicine from a vet to effectively treat either. The symptoms of mange - hairloss, itchiness, scabs, discolouration - may have other causes. These can include fleas, other parasites, bacteria, fungus, or allergies. You really need to seek veterinary attention to diagnose what the problem is before you start treatment.