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# Septra (Trimethoprim/Sulfamethoxazole)
## Overview
Septra is a combination antibiotic of trimethoprim and sulfamethoxazole. It is a bacteriostatic agent that inhibits sequential steps in the folic acid synthesis pathway.
## Primary Indications
* Urinary tract infections (UTIs)
* *Pneumocystis jirovecii* pneumonia (PJP) prophylaxis and treatment
* Bacterial prostatitis
* Acute otitis media
* Acute exacerbations of chronic bronchitis
* Shigellosis gastroenteritis
* Traveler's diarrhea (*Enterotoxigenic E. coli*)
## Adult Dosing
* **Uncomplicated Urinary Tract Infection (UTI):**
* 160 mg trimethoprim / 800 mg sulfamethoxazole (1 double-strength tablet) orally every 12 hours for 3 days.
* **Complicated Urinary Tract Infection (UTI) and Prostatitis:**
* 160 mg trimethoprim / 800 mg sulfamethoxazole orally or intravenously every 12 hours for 14 days (UTI) or 28 days (prostatitis).
* ***Pneumocystis jirovecii* Pneumonia (PJP) Treatment:**
* 15-20 mg/kg/day trimethoprim component, divided into 3-4 doses, given IV or PO. Duration is typically 14-21 days.
* ***Pneumocystis jirovecii* Pneumonia (PJP) Prophylaxis:**
* 1 double-strength tablet (160 mg TMP/800 mg SMX) orally once daily.
* Alternatively, 1 double-strength tablet orally 3 times a week.
* **Shigellosis:**
* 160 mg trimethoprim / 800 mg sulfamethoxazole orally every 12 hours for 5 days.
* **Traveler's Diarrhea:**
* 160 mg trimethoprim / 800 mg sulfamethoxazole orally every 12 hours for 3-5 days.
Maximum daily dose: 20 mg/kg/day trimethoprim component, divided every 6 or 12 hours.
## Pediatric Dosing
Dosing is based on the trimethoprim component (TMP) and is generally calculated per kg of body weight.
* **Urinary Tract Infection (UTI):**
* 8 mg/kg/day TMP divided into 2 doses, given orally every 12 hours.
* For children >12 years: 160 mg TMP / 800 mg SMX (1 DS tablet) PO BID for 3 days.
* ***Pneumocystis jirovecii* Pneumonia (PJP) Treatment:**
* 15-20 mg/kg/day TMP divided into 3-4 doses, given IV or PO. Duration is typically 14-21 days.
* ***Pneumocystis jirovecii* Pneumonia (PJP) Prophylaxis:**
* 5 mg/kg/day TMP divided into 2 doses, given orally every 12 hours.
* Alternatively, 7.5 mg/kg/day TMP divided into 2 doses, given orally every 12 hours, 3 times a week.
*Note: Pediatric dosing of sulfamethoxazole is typically based on a TMP:SMX ratio of 1:5.*
## Dose Adjustments
* **Renal Impairment:**
* **CrCl > 30 mL/min:** No adjustment needed.
* **CrCl 15-30 mL/min:** Reduce maintenance dose by 25-50%.
* **CrCl < 15 mL/min:** Avoid use or give only for life-threatening situations with careful monitoring; dose should not exceed 2.5 mg/kg/day TMP.
## Contraindications
* Known hypersensitivity to trimethoprim, sulfamethoxazole, or sulfonamides.
* Documented megaloblastic anemia due to folate deficiency.
* Infants less than 2 months of age (due to risk of kernicterus).
* Severe renal or hepatic insufficiency when pharmacologic effect cannot be monitored.
* History of drug-induced thrombocytopenia with sulfonamides or trimethoprim.
## Adverse Effects
Common: Rash, nausea, vomiting, diarrhea, hyperkalemia.
Serious: Severe cutaneous reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis), severe hypersensitivity reactions (anaphylaxis), Stevens-Johnson syndrome, aplastic anemia, agranulocytosis, thrombocytopenia, leukopenia, hepatitis, cholestatic jaundice, crystalluria, hyperkalemia, hyponatremia, hypoglycemia.
## Key Drug Interactions
* **Warfarin:** Increased INR. Monitor INR closely and adjust warfarin dose.
* **ACE Inhibitors/ARBs:** Increased risk of hyperkalemia.
* **Potassium-sparing diuretics:** Increased risk of hyperkalemia.
* **Methotrexate:** Increased risk of methotrexate-induced myelosuppression.
* **Cyclosporine:** Increased cyclosporine levels and nephrotoxicity.
* **Digoxin:** Increased digoxin levels.
* **Potassium supplements:** Increased risk of hyperkalemia.
* **Rifampin:** May decrease levels of TMP/SMX.
* **Sulfonylureas:** Increased risk of hypoglycemia.
* **Tricyclic Antidepressants (TCAs):** Trimethoprim may inhibit CYP2D6, potentially increasing TCA levels.
## Monitoring
* **Renal function:** Baseline and periodic assessment of BUN and creatinine, especially in patients with impaired renal function.
* **Complete blood count (CBC) with differential:** Baseline and periodic assessment for evidence of hematologic toxicity. Discontinue if significant reduction in blood cell counts occurs.
* **Electrolytes:** Particularly potassium, especially in patients with renal impairment or those taking other medications that affect potassium.
* **Liver function tests (LFTs):** Periodic assessment, especially in patients with a history of liver disease.
* **Therapeutic drug monitoring:** May be considered for PJP treatment in certain situations, though not routinely performed.
* **Signs of hypersensitivity/cutaneous reactions:** Counsel patients to report rash or other signs of allergic reaction immediately.
## Clinical Pearls
* Administer oral doses with a full glass of water.
* Encourage adequate fluid intake to prevent crystalluria.
* Use with caution in patients with G6PD deficiency due to risk of hemolytic anemia.
* Consider concurrent folic acid supplementation (5 mg daily) in patients receiving long-term therapy to reduce the risk of hematologic side effects, especially in elderly patients or those with pre-existing folate deficiency.
* For IV administration, dilute in a compatible solution (e.g., D5W) and infuse over 30-60 minutes. Do not infuse rapidly as it can cause local reactions.
* Co-trimoxazole is a component of TEM (Trimethoprim-Sulfamethoxazole, Erythromycin, Metronidazole) regimen for bacterial vaginosis, though this is off-label.
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*This information is intended for clinical use and does not replace professional medical advice. Always consult the most current prescribing information and institutional guidelines before administering any medication.*
