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# Septran (co-Trimoxazole)
## Overview
Fixed combination of trimethoprim (TMP) and sulfamethoxazole (SMX) in a 1:5 ratio (e.g., 80 mg TMP/400 mg SMX per single-strength tablet; 160/800 mg per double-strength [DS] tablet). Available orally and intravenously. Bacteriostatic; inhibits sequential steps in folate synthesis.
## Primary Indications
- Urinary tract infections (UTI)
- Acute otitis media
- Respiratory infections (including PCP prophylaxis/treatment)
- Traveler's diarrhea
- Shigellosis
- Nocardiosis, toxoplasmosis (often in combination)
## Adult Dosing
- Standard infections (UTI, bronchitis, otitis): 1 DS tablet (160/800 mg) PO q12h.
- Pneumocystis jirovecii pneumonia (PCP) treatment: 15–20 mg/kg/day TMP divided q6–8h (IV or PO). Common regimen: 5 mg/kg TMP (≈25 mg/kg SMX) q6–8h; max 20 mg/kg/day TMP.
- PCP prophylaxis: 1 DS tablet daily or 3 times weekly (per local guideline).
- Max daily dose: TMP 20 mg/kg/day; SMX 100 mg/kg/day (limited by toxicity).
## Pediatric Dosing
Dose expressed as TMP component unless noted.
- UTI / otitis media: 8 mg/kg/day TMP (40 mg/kg/day SMX) divided q12h. Max: 320 mg TMP/day (i.e., 2 DS tablets/day).
- PCP treatment: 15–20 mg/kg/day TMP divided q6–8h (IV or PO). Max 20 mg/kg/day TMP.
- PCP prophylaxis: 5 mg TMP/kg daily or 3 times weekly (max 320 mg TMP/day). For children >40 kg, use adult dose.
- Exact dosing may vary by local protocol (e.g., prophylaxis schedule, weight bands).
## Dose Adjustments
- **Renal**: CrCl 15–30 mL/min: reduce dose by 50% (e.g., use single-strength q12h or extend interval to q18–24h). CrCl <15 mL/min: contraindicated (unless on dialysis, with monitoring).
- **Hepatic**: avoid in severe hepatic disease; caution in mild–moderate.
## Contraindications
- Severe renal impairment (CrCl <15 mL/min)
- Severe hepatic damage
- Megaloblastic anemia (due to folate deficiency)
- Known hypersensitivity to sulfonamides or TMP
- Pregnancy (especially near term) and lactation (risk of kernicterus in newborn; avoid if G6PD deficiency, prematurity, hyperbilirubinemia)
## Adverse Effects
- Rash, photosensitivity, GI upset (nausea, vomiting)
- Hyperkalemia (especially with IV, in elderly, renal impairment)
- Hematologic: thrombocytopenia, agranulocytosis, hemolytic anemia (G6PD deficiency)
- Severe: Stevens-Johnson syndrome, toxic epidermal necrolysis, cholestatic jaundice
- Crystalluria (prevent with adequate hydration)
- Hypersensitivity reactions (more common in HIV)
## Key Drug Interactions
- Warfarin: increased INR (monitor closely)
- Methotrexate: increased methotrexate toxicity (folate antagonism)
- ACE inhibitors / ARBs: increased risk of hyperkalemia
- Phenytoin: elevated phenytoin levels
- Sulfonylureas: enhanced hypoglycemic effect
- Thiazide diuretics: increased risk of thrombocytopenia (elderly)
## Monitoring
- CBC (baseline and periodically, especially in prolonged therapy)
- Renal function and serum potassium (especially with IV, in elderly, renal insufficiency)
- Urinalysis (for crystals)
- Signs of hypersensitivity or severe skin reactions
## Clinical Pearls
- Hydrate well (≥2 L/day) to prevent crystalluria.
- Hypersensitivity incidence is higher in HIV-infected patients.
- **Not recommended** for group A streptococcal pharyngitis due to high resistance.
- Resistance patterns vary; check local antibiogram.
- IV formulation must be diluted (e.g., in D5W) and infused over 60–90 minutes; avoid rapid bolus.
- Caution in elderly, renal impairment, and those on potassium-sparing drugs (hyperkalemia risk).
*This information is an educational summary and does not replace full prescribing guidelines. Always verify current dosing, indications, and safety data from authoritative references (e.g., national formulary, local protocols, full prescribing information) before clinical use.*
