Sulfamethoxazole / trimethoprim
"I got this for a UTI at the ER. The next day I had an intense sharp headache that caused me to throw up. I lost my appetite as well and hardly ate for two weeks because I couldn't stomach anything and ended up losing a lot of weight . I also broke out in a terrible painful rash. It was all over my arms, chest, face, and neck. After two days I had to go to my regular doctor for steroids and a change in the antibiotics. The absolute worst experience on any medication, really surprised it's still on the market after reading all the reviews"