Common side effects of intravenous sulfamethoxazole and trimethoprim use include sun sensitivity, itching, head pain, rash and loss of appetite, according to WebMD. Other possible side effects of oral sulfamethoxazole and trimethoprim use that are considered rare include abdominal or stomach pain, black tarry stools, blistering, peeling or loosening of skin, chest pain, chills, cough or hoarseness, dark urine, diarrhea, dizziness, fever, tiredness or weakness, according to the Mayo Clinic.
Additional side effects associated with the oral form of sulfamethoxazole and trimethoprim are shortness of breath, changes in skin color, vomiting of blood, joint or muscle pain, loss of appetite, nausea, rash and wheezing, according to the Mayo Clinic.
The combination of sulfamethoxazole and trimethoprim in oral form is an antibiotic used to treat urinary-tract infections, middle-ear infections, bronchitis, traveler's diarrhea and bacillary dysentery, according to the Mayo Clinic. It is also used to treat serious forms of pneumonia that normally occur in patients with weakened immune systems. The drug is available by prescription only, and it does not work as effectively in patients with a history of alcohol abuse, folate deficiency, kidney disease, anemia, asthma, diabetes or liver disease. U.S. brands that commonly manufacture the drug include Bactrim, Septra and Sulfatrim.