Trimethoprim/Sulfamethoxazole Drug Profile

What is Trimethoprim/Sulfamethoxazole?

Trimethoprim/sulfamethoxazole is a combination antibiotic medication that belongs to the class of folate synthesis inhibitors. It combines two antimicrobial agents that work synergistically to treat various bacterial infections. This fixed-dose combination is commonly prescribed for urinary tract infections, respiratory infections, and certain other bacterial conditions.

Uses

Trimethoprim/sulfamethoxazole is approved for several bacterial infections:

• Urinary tract infections – Including cystitis and pyelonephritis caused by susceptible bacteria such as E. coli
• Pneumocystis jirovecii pneumonia – Both treatment and prevention, particularly in immunocompromised patients
• Acute exacerbations of chronic bronchitis – When caused by susceptible strains of Streptococcus pneumoniae or Haemophilus influenzae
• Traveler’s diarrhea – Caused by enterotoxigenic E. coli when antibiotics are indicated
• Shigellosis – Treatment of shigella infections when antimicrobial therapy is warranted

How it works

The combination works by blocking two sequential steps in bacterial folate synthesis, which is essential for DNA production and cell division. Sulfamethoxazole inhibits an earlier step by mimicking para-aminobenzoic acid, while trimethoprim blocks a later enzyme in the pathway. This dual mechanism creates a synergistic effect that is more potent than either drug alone and helps prevent bacterial resistance.

Side effects

Common side effects include:

Submit Your Paper

• Nausea and vomiting
• Loss of appetite
• Skin rash
• Diarrhea
• Headache
• Dizziness
• Fatigue
• Elevated potassium levels

Serious side effects to watch for:

• Stevens-Johnson syndrome or toxic epidermal necrolysis (severe skin reactions)
• Blood disorders including severe anemia or low white blood cell counts
• Severe diarrhea that may indicate C. difficile infection
• Liver injury with jaundice or elevated liver enzymes
• Severe allergic reactions with difficulty breathing or swelling

Warnings and precautions

• Sulfa allergy: Patients with known hypersensitivity to sulfonamides should avoid this medication due to risk of severe allergic reactions
• Pregnancy considerations: Generally avoided during pregnancy, especially in the first trimester and near delivery, due to potential risks to the developing fetus
• Kidney function: Dosage adjustment required in patients with impaired kidney function; regular monitoring of kidney function recommended during treatment
• Blood monitoring: Periodic blood tests may be necessary during prolonged treatment to monitor for blood cell count changes
• Folate deficiency: Caution in elderly patients or those with poor nutrition, as the medication can worsen folate deficiency

Interactions

• Warfarin: May enhance anticoagulant effects, increasing bleeding risk and requiring INR monitoring
• Methotrexate: Increased risk of methotrexate toxicity due to reduced elimination
• ACE inhibitors and ARBs: Combined use may lead to dangerous elevation of potassium levels
• Phenytoin: May increase phenytoin levels, potentially causing toxicity
• Digoxin: Can increase digoxin concentrations, requiring dose adjustment and monitoring
• Sulfonylureas: May enhance hypoglycemic effects of diabetes medications
• Cyclosporine: Increased risk of kidney toxicity when used together
• Dofetilide: Contraindicated combination due to increased risk of serious heart rhythm abnormalities

Check interactions with the GMJ Interaction Checker

Dosage

Typical adult dosages vary based on the condition being treated. For most infections, the standard dose is one double-strength tablet (800mg sulfamethoxazole/160mg trimethoprim) every 12 hours for 10-14 days. Urinary tract infections may require shorter courses of 3-5 days. For Pneumocystis pneumonia prevention, lower doses taken three times weekly are common. Dosage adjustments are necessary for patients with kidney impairment. Healthcare providers determine the appropriate dose and duration based on the specific infection, patient factors, and response to treatment.

Sources: FDA DailyMed drug labels (public domain), BNF, WHO Essential Medicines List. This article is for informational purposes only and does not constitute medical advice. Always consult your healthcare provider before starting or stopping any medication.

This work is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are free to share and adapt this content with attribution to GMJ News (news.gmj.ge).