What is the oral liquid equivalent of Septra (trimethoprim and sulfamethoxazole) 1 double-strength tablet twice daily for 3 days?

Converting Septra Double-Strength Tablets to Oral Liquid Formulation

For a prescription equivalent to Septra (trimethoprim-sulfamethoxazole) 1 double-strength tablet twice daily for 3 days, prescribe trimethoprim-sulfamethoxazole oral suspension 40 mg/8 mg per mL, with a dose of 20 mL twice daily for 3 days.

Understanding the Conversion

When converting from tablets to liquid formulation, it's essential to maintain the same active ingredient dosage:

• A double-strength (DS) tablet of trimethoprim-sulfamethoxazole contains:

  • 160 mg trimethoprim
  • 800 mg sulfamethoxazole 1

• The standard oral suspension contains:

  • 40 mg trimethoprim per 5 mL
  • 200 mg sulfamethoxazole per 5 mL

Calculation Process

1. Determine tablet strength:

   • 1 DS tablet = 160 mg trimethoprim + 800 mg sulfamethoxazole

2. Determine liquid concentration:

   • Oral suspension = 40 mg trimethoprim + 200 mg sulfamethoxazole per 5 mL
   • This equals 8 mg trimethoprim + 40 mg sulfamethoxazole per 1 mL

3. Calculate equivalent volume:

   • To deliver 160 mg trimethoprim (equivalent to 1 DS tablet):
     • 160 mg ÷ 8 mg/mL = 20 mL

Prescription Details

Write the prescription as:

Trimethoprim-sulfamethoxazole oral suspension 40 mg/8 mg per mL
Sig: Take 20 mL by mouth twice daily for 3 days
Dispense: 120 mL

Important Administration Instructions

• Shake the suspension well before measuring 2
• Use a calibrated measuring device for accurate dosing
• Administer with food to decrease gastrointestinal side effects
• Ensure adequate hydration during therapy
• If taste is an issue, can be mixed with a small amount of juice or flavored syrup

Clinical Considerations

• This 3-day regimen is appropriate for uncomplicated urinary tract infections, as supported by guidelines 3
• For other indications such as skin infections or respiratory infections, longer durations may be required
• For pediatric patients, dosing would be weight-based at 8-12 mg/kg/day of the trimethoprim component 2

Cautions

• Not recommended for use in patients with severe renal impairment (creatinine clearance <15 mL/min) 1
• Avoid in pregnant patients, especially in the third trimester 2
• Monitor for rare but serious adverse reactions including Stevens-Johnson syndrome 2

This conversion maintains therapeutic equivalence while providing the medication in a liquid form that may be easier for certain patients to take.