I've heard Benadryl is safe for dogs in small doses. Best to check with the vet to see about what a safe dose is for your dogs weight and if it even works on tapeworm. Apparently tapeworms are attracted to milk and you can expel them and their larvae and kill them by giving your dog a milk bath. http://www.grandmas-wisdom.com/tapeworm.html
Did the vet tell you to treat the tapeworms with Benadryl? I know you can give Benadryl to dogs for allergies since the vet said to do that with my dog. If the vet said it would be ok, then I have to assume it'll would be fine for tapeworms too. Otherwise get the right medication.
So. You want to give an allergy medicine to treat tapeworms. Maybe try some M&M's. I hear that they are good for the heart. Cough syrup does wonders for a dogs hair falling out. Chicken is a cure for bad dog breath. Ridiculous.