Trimethoprim/sulfamethoxazole (TMP/SMX), also known as Septra, is a commonly encountered and prescribed antibiotic in emergency department patients. The side effects associated with TMP/SMX are generally mild and self-limited, but serious side effects, including Stevens-Johnson syndrome and drug-induced aseptic meningitis, have been reported. We discuss the case of a 33-year-old woman who presented to our emergency department with the signs and symptoms of meningeal inflammation after being prescribed TMP/SMX 3 days earlier for an abscess with cellulitis.