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## Septra (Trimethoprim/Sulfamethoxazole) ## Overview Septra is a combination antibiotic containing trimethoprim (TMP) and sulfamethoxazole (SMX) in a 1:5 ratio. It works by inhibiting sequential steps in the folic acid synthesis pathway, which is essential for bacterial growth. ## Primary Indications * Urinary tract infections (UTIs) due to susceptible organisms. * Pneumocystis jirovecii pneumonia (PCP) prophylaxis and treatment. * Shigellosis enteritis. * Acute otitis media. * Acute exacerbations of chronic bronchitis. * Traveler's diarrhea. * Prophylaxis and treatment of certain other bacterial infections as per local guidelines. ## Adult Dosing * **Uncomplicated UTIs:** 1 DS (double strength) tablet (800 mg SMX / 160 mg TMP) every 12 hours for 3 days. * **PCP Treatment:** 15 mg/kg TMP equivalent (divided into 3-4 doses) per day, given orally or IV. This typically translates to 2 DS tablets every 8 hours for 14-21 days. * **PCP Prophylaxis:** 1 DS tablet (800 mg SMX / 160 mg TMP) daily, or 1 DS tablet 3 times a week (on alternate days). * **Other Indications:** Dosing varies; common adult doses range from 1 DS tablet every 12 hours to 2 DS tablets every 12 hours. Refer to specific guidelines for duration and dose. ## Pediatric Dosing * Dosing is based on the TMP component. * **General Infections:** 8 mg/kg TMP equivalent (divided into 2 doses) per day, given orally or IV. This is approximately 10 mg/kg/day of the SMX component. * **PCP Treatment:** 15 mg/kg TMP equivalent (divided into 3-4 doses) per day, given orally or IV. * **PCP Prophylaxis:** 5 mg/kg TMP equivalent (divided into 2 doses) per day, given orally. * Maximum daily dose in children is generally 320 mg TMP (equivalent to 4 DS tablets). ## Dose Adjustments * **Renal Impairment (CrCl):** * CrCl > 30 mL/min: No adjustment needed. * CrCl 15-30 mL/min: Give usual dose every 18 hours. * CrCl < 15 mL/min: Avoid use or give half the usual dose every 18 hours. ## Contraindications * Known hypersensitivity to trimethoprim, sulfonamides, or any component of the formulation. * Documented megaloblastic anemia due to folate deficiency. * Infants less than 2 months of age (risk of kernicterus). * Severe renal or hepatic insufficiency when pharmacologic effect cannot be monitored. * History of drug-induced thrombocytopenia with either component. ## Adverse Effects * **Common:** Nausea, vomiting, rash, urticaria, hyperkalemia. * **Serious:** Severe skin reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis), bone marrow suppression (anemia, leukopenia, thrombocytopenia), hepatic injury, hyperkalemia, renal impairment, crystalluria. ## Key Drug Interactions * **ACE inhibitors/ARBs/Potassium-sparing diuretics/Potassium supplements:** Increased risk of hyperkalemia. * **Warfarin:** Increased INR. Monitor INR closely. * **Methotrexate:** Increased methotrexate levels and toxicity. * **Cyclosporine:** Increased cyclosporine levels, particularly in renal transplant patients. * **Thiazide diuretics:** Increased risk of thrombocytopenia in elderly patients. * **Potent CYP2C9 inhibitors:** May increase SMX levels. ## Monitoring * Renal function (BUN, creatinine). * Complete blood counts (CBC) with differential, especially with prolonged therapy or in immunocompromised patients. * Electrolytes (especially potassium). * Liver function tests. * Signs of hypersensitivity reactions. * For patients on warfarin, monitor INR regularly. ## Clinical Pearls * Encourage adequate fluid intake to prevent crystalluria. * Monitor for signs of hypersensitivity, especially in patients with a history of sulfa allergy. * Be aware of increased risk of hyperkalemia, particularly in elderly patients or those with renal impairment. * Rash does not always indicate hypersensitivity and can be a sign of more severe reactions. **Disclaimer:** This information is intended for healthcare professionals and is not a substitute for professional medical advice. Always consult the most current prescribing information and institutional protocols for complete details.