Trimethoprim-sulfamethoxazole is a commonly used antibiotic for uncomplicated urinary tract infections caused by susceptible bacteria, particularly E. coli. It works by blocking bacterial folate synthesis at two sequential steps, which makes it highly effective when resistance rates are low. TMP-SMX is often considered a first-line treatment option in areas where local E. coli resistance is less than 20%. Potential side effects include gastrointestinal upset, rash, and, rarely, more serious reactions such as Stevens-Johnson syndrome or blood count abnormalities, so it should be used cautiously in patients with sulfa allergies or certain medical conditions.