Sulfamethoxazole / trimethoprim
"I was prescribed sulfameth for a gross bacterial infection (jacuzzi rash).  While on it I experienced headaches, foul gas, loss of appetite, constant thirst, extreme fatigue, dizziness, spotting, diarrhea (should have kept up with my probiotics).  On the eighth day I had itchy hives ALL over my body, severe lower back pain, neck & shoulder pain & stiffness.  Dr. said to stop medicine & consider myself allergic to sulfa drugs.  My original rash had cleared up but so did my two children who didn't have any antibiotics.  For me, obviously not worth the side effects."