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Diphenhydramine (Benadryl) can safely be given to dogs for allergic reactions. You can give 25mg by mouth for her weight. I do have to tell you that if she is having difficulty breathing, she should probably see a veterinarian right away to get some injectable medications. Its a bit unusual that both of her back legs are swollen - we usually do not see such a severe reaction. I'm wondering if there could be something else going on.
When giving the Benadyrl, make sure its just the Diphenhydramine, no other ingredients.