Bactrim Dosing for Pneumocystis Prophylaxis in Patients on Chronic Steroids
For patients on chronic steroids requiring Pneumocystis jirovecii pneumonia (PCP) prophylaxis, use Bactrim DS (800mg/160mg) one tablet daily, or alternatively, one DS tablet three times weekly on consecutive days. 1, 2, 3
Standard Prophylactic Dosing Regimens
The FDA-approved dosing for PCP prophylaxis offers two options 2, 3:
- Daily regimen: 1 double-strength tablet (800mg sulfamethoxazole/160mg trimethoprim) once daily 1, 2
- Three-times-weekly regimen: 1 double-strength tablet three times per week on consecutive days (e.g., Monday-Tuesday-Wednesday) 1
Both regimens are equally effective for prophylaxis, with the three-times-weekly schedule offering reduced drug exposure and potentially fewer side effects while maintaining efficacy 1.
Critical Drug Interaction with Methotrexate
If the patient is taking methotrexate concurrently, use only the prophylactic dose (single-strength or double-strength once daily or three times weekly), NOT treatment doses. 4, 1
- Treatment doses of Bactrim (800mg/160mg twice daily) combined with methotrexate carry significant risk of severe bone marrow suppression 4, 1
- The lower prophylactic doses are generally tolerated when used with methotrexate, but monitoring is essential 4
- Monthly complete blood count monitoring is recommended for patients on this combination to detect hematological toxicity, particularly thrombocytopenia 1
Renal Dose Adjustment
For patients with impaired renal function 2, 3:
- CrCl >30 mL/min: Standard prophylactic dosing (no adjustment needed)
- CrCl 15-30 mL/min: Reduce to half the usual regimen
- CrCl <15 mL/min: Use not recommended
- Hemodialysis patients: 500mg three times weekly after dialysis 1
Note that trimethoprim can increase serum creatinine by competitively inhibiting tubular secretion without actually decreasing glomerular filtration rate—this is a benign effect and does not represent true renal dysfunction 5.
Important Safety Monitoring
Regular monitoring is essential for patients on chronic Bactrim prophylaxis 1:
- Obtain baseline complete blood count before initiating therapy
- Monitor CBC monthly for bone marrow suppression (thrombocytopenia, leukopenia, anemia)
- Watch for serious hypersensitivity reactions including Stevens-Johnson syndrome and toxic epidermal necrolysis
- Avoid use in patients with G6PD deficiency due to hemolytic anemia risk 1
Context for Chronic Steroid Use
Patients on chronic steroids are at increased risk for PCP, particularly when receiving prednisone ≥20mg daily for ≥1 month or equivalent doses of other corticosteroids 4. The immunosuppressive effects of glucocorticoids necessitate prophylaxis, and screening for toxicities of both glucocorticoid and Bactrim use should be conducted regularly 4.