Activated charcoal can act as an adsorbent (kind of a "chemical sponge") to bind toxins in the gut. If you use a preparation that is safe for people and use a smaller dose, it should be harmless.
If the problem was an insect sting, I doubt that there are any toxins in the gut that the charcoal would help with.
So many times we are unable to determine the source of the allergen.
Kenalog is a long-acting corticosteroid (cortisone) injection that stays in the body for weeks, where the dexamethasone lasts only a couple of days.
Antihistamines like benadryl are often helpful for something like an insect sting, even though most dogs don't get much relief from them in regard to allergic itching.
Are your dog's ears swollen, inflated, squishy, almost resembling a marshmallow? Has your dog been consistently shaking his head, scratching his ear and keeping his head tilted to the side? If you answered yes to any of the above, chances are high your dog may have developed an unsightly condition known as aural hematoma.
What is aural hemtoma? The technical term may be a bit intimidating, but "aural" simply means ear, while "hematoma" means "blood has accumulated inside the ear". If you look carefully at your dog's ears, you will likely see that the actual swollen part is the underlying surface of the pinna. Upon palpation most dogs will display a pain signal.