What It Is

Sulfamethoxazole and trimethoprim are two antimicrobial agents combined in a fixed-dose formulation to enhance their efficacy. This combination is commonly used to treat a variety of bacterial infections. Sulfamethoxazole is a sulfonamide antibiotic, while trimethoprim is a dihydrofolate reductase inhibitor. Together, they form a potent synergistic antibiotic known as co-trimoxazole or sulfamethoxazole-trimethoprim (SMX-TMP).

Dosage Forms and Strengths

Sulfamethoxazole and trimethoprim are available in several dosage forms and strengths:

1. Oral Tablets:

   • Standard tablets: 400 mg sulfamethoxazole and 80 mg trimethoprim.
   • Double strength tablets: 800 mg sulfamethoxazole and 160 mg trimethoprim.

2. Oral Suspension:

   • Typically contains 200 mg sulfamethoxazole and 40 mg trimethoprim per 5 mL.

3. Injectable Form:

   • Available in intravenous (IV) formulations for hospital use, with similar concentrations to oral tablets.

Brand and Generic Names

Brand Names:

• Bactrim
• Septra

Generic Names:

• Sulfamethoxazole and trimethoprim

Indications for Use

Sulfamethoxazole and trimethoprim are indicated for the treatment of various infections caused by susceptible organisms. Common indications include:

• Urinary Tract Infections (UTIs): Effective against both uncomplicated and complicated UTIs.
• Respiratory Tract Infections: Includes bronchitis, otitis media, and sinusitis.
• Gastrointestinal Infections: Particularly useful for traveler's diarrhea and certain types of gastroenteritis.
• Pneumocystis jirovecii Pneumonia (PCP): Used for both the treatment and prophylaxis in immunocompromised patients.
• Certain types of skin infections: Including acne and cellulitis.

Mechanism of Action

Sulfamethoxazole and trimethoprim work synergistically to inhibit bacterial folic acid synthesis.

• Sulfamethoxazole: It competes with para-aminobenzoic acid (PABA), a precursor in the bacterial synthesis of folate. This inhibition prevents bacteria from producing folic acid, which is essential for their growth and replication.
• Trimethoprim: It inhibits dihydrofolate reductase, an enzyme that catalyzes the reduction of dihydrofolate to tetrahydrofolate, another critical step in folate synthesis. The combination of these two mechanisms results in a more potent antibacterial effect.

Administration and Dosage

The dosage of sulfamethoxazole and trimethoprim depends on the infection being treated, the patient’s age, and renal function.

• Adult Dosage:

  • For uncomplicated UTIs: 160 mg trimethoprim/800 mg sulfamethoxazole every 12 hours for 10 to 14 days.
  • For PCP prophylaxis: 160 mg trimethoprim/800 mg sulfamethoxazole once daily.

• Pediatric Dosage:

  • Generally dosed based on body weight, such as 6 to 12 mg/kg/day of trimethoprim component, divided into two doses.

• Administration: The medication can be taken orally with or without food. For IV formulations, administration should be done under medical supervision.

Side Effects and Precautions

Side Effects:

• Common: Nausea, vomiting, rash, and diarrhea.
• Serious: Allergic reactions, severe skin reactions (Stevens-Johnson syndrome), hematological abnormalities (e.g., anemia, leukopenia), liver toxicity, and kidney impairment.

Precautions:

• Allergy History: Caution in patients with a history of sulfonamide or trimethoprim allergy.
• Renal Function: Adjust dosage in patients with renal impairment to avoid toxicity.
• Hematological Monitoring: Regular monitoring of blood counts is advised, especially with prolonged therapy.
• Pregnancy: Generally avoided during the first trimester and caution used during the second and third trimesters due to potential risks to the fetus.

Conclusion

Sulfamethoxazole and trimethoprim remain a cornerstone in the treatment of various bacterial infections due to their synergistic action and broad-spectrum efficacy. While generally safe and effective, it is crucial to be aware of potential side effects and contraindications. Proper dosing, monitoring, and adherence to precautions ensure optimal outcomes and minimize risks associated with this antimicrobial combination.

INDICATION AND DOSAGE OF