How to Write a Prescription for Bactrim Three Times Weekly
For Pneumocystis prophylaxis, prescribe trimethoprim-sulfamethoxazole (Bactrim) 160/800 mg (one double-strength tablet) three times weekly on consecutive days (e.g., Monday-Tuesday-Wednesday). 1
Prescription Writing Format
Write the prescription as follows:
Trimethoprim-sulfamethoxazole DS (160 mg/800 mg)
- Sig: Take 1 tablet by mouth three times weekly on consecutive days (Monday, Tuesday, Wednesday)
- Dispense: #12 tablets (for 1 month supply)
- Refills: As appropriate for clinical situation
Clinical Context and Indications
This three-times-weekly dosing regimen is specifically recommended for:
- Pneumocystis jiroveci pneumonia (PCP) prophylaxis in HIV-infected adults with CD4+ counts <200 cells/µL 2
- Alternative prophylactic schedule when daily dosing is not tolerated or preferred 1
The three-consecutive-days-per-week schedule provides adequate prophylactic coverage while potentially reducing adverse effects compared to daily dosing 1.
Important Dosing Considerations
Standard Daily Dosing Alternative
If three-times-weekly dosing is inadequate or the clinical situation requires daily prophylaxis, the standard regimen is one double-strength tablet (160/800 mg) once daily, 7 days per week 2, 3, 4.
Renal Dose Adjustment
For patients with impaired renal function requiring three-times-weekly dosing:
- CrCl >30 mL/min: Standard three-times-weekly regimen 2, 3, 4
- CrCl 15-30 mL/min: Consider reducing to half the usual regimen 3, 4
- CrCl <15 mL/min: Use not recommended 3, 4
For patients on hemodialysis requiring prophylaxis, administer 500 mg three times weekly after dialysis 2.
Critical Safety Warnings
Contraindications to Screen For
- G6PD deficiency: Absolute contraindication due to hemolytic anemia risk 1
- Pregnancy at term: Avoid use in late pregnancy 1
- Age <2 months: Contraindicated in infants 3, 4
Monitoring Requirements
- Obtain baseline complete blood count before initiating therapy 1
- Monitor monthly for hematologic toxicity, particularly thrombocytopenia 1
- Assess for drug interactions, especially with methotrexate at treatment doses (avoid concurrent use due to severe bone marrow suppression risk) 1
Common Pitfalls to Avoid
Do not write "three times a week" without specifying consecutive days - the evidence supports consecutive-day dosing (e.g., Mon-Tue-Wed), not scattered throughout the week 1
Do not assume all three-times-weekly regimens are equivalent - this specific schedule is validated for prophylaxis, not for treatment of active infections 1, 3, 4
Do not use this regimen for acute treatment - active PCP requires 75-100 mg/kg sulfamethoxazole and 15-20 mg/kg trimethoprim per 24 hours divided every 6 hours for 14-21 days 3, 4
Verify the indication - three-times-weekly dosing is appropriate for prophylaxis but inadequate for treating active infections 2, 1, 3, 4