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.
I am sorry to hear of what everyone has gone through but am relieved to know that I am not the only one. I usually cannot take oral steroids because they make me feel weird and sick. But due to severe tendinitis pain I had a cortizone shot in one wrist in Jan 2013 & a shot in the other wrist March 2013. I already had the racing heart after the 1st shot but did not realize how much worse it could get until after the second shot. I already have high blood pressure, irregular heartbeat and I take two blood pressure meds and a low dose anti anxiety med to help with mild anxiety and RLS. The racing heart issue would actually prevent me from going to sleep and/or wake me up because my heart was suddenly slamming in my chest. Sometimes it would take my breath away. It has been very scary and it has taken months for the effect to wear off. Unfortunately now my hand/wrist pains are coming back but I do NOT want to have those shots ever again. I hope anyone who is still going through the side effects feels better soon.