Trimethoprim-sulfamethoxazole is a combination antibiotic used to treat various bacterial infections. It works by inhibiting the production of folic acid in bacteria, which is essential for their growth and reproduction. This medication is effective against a range of infections, including urinary tract infections and certain types of pneumonia. This antibiotic is often prescribed in tablet form or as an intravenous solution. Common side effects may include nausea, vomiting, and skin rashes. It is important to take the full course of the medication as prescribed to ensure the infection is completely cleared and to prevent antibiotic resistance.