Leukocytoclastic Vasculitis and Bactrim.
QUESTION: What’s up with Bactrim?
Bactrim (TMP-SMX)
We use it for a wide range of infections—UTIs, certain cases of traveler’s diarrhea, MRSA skin and soft tissue infections (SSTIs), Pneumocystis jirovecii pneumonia (PJP), and toxoplasmosis. In the ED, we most commonly prescribe it for UTIs and purulent cellulitis or other SSTIs in patients with MRSA risk factors.
Mechanism
Trimethoprim and sulfamethoxazole act synergistically by inhibiting folate production in microorganisms.
Common Side Effects
Like other antibiotics: nausea, vomiting, fatigue, and loss of appetite.
Serious Adverse Effects
Bactrim is also known for some more concerning risks:
Stevens–Johnson syndrome (SJS)
Hematologic: anemia, agranulocytosis, myelosuppression
GI/Renal: C. diff, interstitial nephritis, renal tubular acidosis, hyperkalemia
Hepatotoxicity
Hemolytic anemia in patients with G6PD deficiency
Anaphylaxis in patients with sulfa allergy
Pregnancy and Breastfeeding
Category D: avoid in pregnancy due to antifolate activity.
Breastfeeding: excreted in small amounts; avoid if the infant has G6PD deficiency.
Question: What is Leukocytoclastic Vasculitis?
One of the rarer complications of Bactrim is leukocytoclastic vasculitis.
Definition: Immune-complex–mediated small-vessel vasculitis.
Etiology: Idiopathic in ~50% of cases. Most often triggered by infections or drugs.
Presentation: Typically develops 1–3 weeks after the trigger. Classically presents with erythematous macules and palpable purpura bilaterally on dependent areas (legs, buttocks). Unilateral or localized lesions are rare. Other findings may include hemorrhagic vesicles/bullae, pustules, nodules, ulcers, or livedo reticularis.
Systemic symptoms: Less common, but may include low-grade fever, malaise, weight loss, myalgias, and arthralgias.
Course/Management:
Most cutaneous cases are mild and resolve with supportive care: leg elevation, rest, compression stockings, antihistamines.
Chronic or refractory disease may require a 4–6 week steroid taper.
Mortality is low (~2%), and usually related to systemic disease. ~90% of patients have spontaneous resolution within weeks to months.
Always address the underlying cause—treat infection or discontinue the offending drug.
References
Kemnic TR, Coleman M. Trimethoprim Sulfamethoxazole. [Updated 2022 Nov 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK513232/
Baigrie D, Crane JS. Leukocytoclastic Vasculitis. [Updated 2023 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482159/
