Hi, thanks for the query to HCM.
is ingredient of benadryl
and this is anti allergy
and antihistamine means to prevent sulfa reaction in your's case.
No doubt this benadryl will prevent sulfa reaction.
If your's swelling is due to reaction ( allergy) this will subside .
If it is due to another reason means infection or inflammation than there is no chances of swelling decreases.
So this is the best way to consult a physician beside a dentist
and decide future course of action in your problem.
Thanks and regards.