Sulfamethoxazole / trimethoprim
"My son had a serious "diaper rash" (even though he was potty trained) for weeks that no creams would clear up. It started out as a few pimples and quickly erupted into large, painful, pussy boils with blisters after breaking open. After trying every cream imaginable, my doctor prescribed Bactrim and the rash was literally better over night. In a few days, it was cleared up completely! He did, however, experience a lack of appetite, lethargy, upset stomach, and diarrhea. Nothing severe, but a little uncomfortable for him...but not as bad as his painful rash had been!"