Study of New Antibiotic Regimen for the Treatment of Uncomplicated Cellulitis in Emergency Department Patients

Inclusion Criteria * Must have cellulitis as defined here: 1. Definition A (preferred definition): Recent onset of soft tissue erythema, considered by the treating clinician to be bacterial in origin, and associated with signs of infection that include at least two of the following: pain, swelling, warmth, fever, lymphangitis, induration, or ulceration. 2. Definition B (ONLY for darkly-pigmented subjects who cannot use Definition A): Recent onset of soft tissue color change, pain, or swelling, considered by the treating clinician to be bacterial in origin, and at least one of the following: warmth, fever, induration, or ulceration * Clinical (non-research) attending physician agrees with treatment with cephalexin until 3 days after all symptoms gone, using our weight-based dosing * Responsible clinical attending physician comfortable with adding trimethoprim-sulfamethoxazole vs. placebo to the above * Subject understands the study and signs written informed consent. * Subject agrees to drink at least 1 liter of fluid per day. * Subject will commit to all follow-up appointments Exclusion Criteria: * Age \< 12 months or weight \<15 kg * Current skin infection has already been treated * Allergy to sulfa drugs * History of severe allergic reaction to penicillin (defined as anaphylactoid reaction, angioedema, bronchospasm) * Current use of any antibiotic (other than topicals) * Diabetes mellitus * Cellulitis complicated by underlying peripheral vascular disease * Renal insufficiency, defined as patient report, clinical suspicion, or creatinine\>1.3 or EGFR\<60 on the last-available set of chemistry results in our computer system * Hospital admission required * Presence of \> 1 cc of purulent discharge at any time * Cellulitis involving an indwelling vascular, enteric, or urinary catheter * Immunocompromise of any etiology * Pregnancy * Breast feeding * Facial cellulitis (infection is above the clavicles) * Cellulitis associated with marine or freshwater injury, or animal or human bite. (Insect bites not excluded.) * History of glucose-6-phosphate dehydrogenase deficiency * Taking coumadin (warfarin), methotrexate, cisapride, phenytoin (dilantin), digoxin, or dofetilide * Known megaloblastic anemia due to folate deficiency.