Sulfamethoxazole / trimethoprim
"I was given dosage of Bactrim for 10 days 2 times a day for a spider bite. Side effects were nausea, diarrhea, tiredness, loopy feeling, slight fever, and loss of appetite. I had a general feeling of malaise and "blah". All I wanted to do was use the bathroom and sleep. I couldn't focus on anything at work.  I couldn't make myself finish it. I stopped after day 5. It didn't matter if I ate a bunch of food or drank a lot of water while taking it. I was running to the bathroom.  I have never had such bad side effects to any antibiotic.  If there are complications to the spider bite, I'll go back to the doctor for something else."