SN instructed patient and caregiver about Bactrim. Bactrim is an antibiotic used to treat certain bacterial infections, including urinary tract infections, ear infections, enteritis, pneumonia, traveler's diarrhea, and episodes of difficulty breathing in people with chronic bronchitis. Serious allergic reactions, such as skin rash, sore throat, fever, joint pain, cough, shortness of breath, paleness, or yellowing of your skin or the whites of your eyes. Low platelet levels that may be life-threatening, but usually go back to normal within a week of stopping treatment with Bactrim. Watery and bloody stools.