What is a Double Dose of Bactrim?
A double-strength (DS) tablet of Bactrim contains 800 mg sulfamethoxazole and 160 mg trimethoprim, which is exactly twice the amount found in a regular-strength tablet (400 mg sulfamethoxazole and 80 mg trimethoprim). 1
Standard Formulations
Bactrim is available in two tablet strengths:
- Regular-strength tablet: 400 mg sulfamethoxazole + 80 mg trimethoprim
- Double-strength (DS) tablet: 800 mg sulfamethoxazole + 160 mg trimethoprim 1
Common Dosing Regimens Using DS Tablets
The term "double dose" can be confusing because it may refer to either the DS tablet formulation itself OR an actual doubled dose for specific indications:
For Most Common Infections
- Standard adult dose: 1 DS tablet every 12 hours 1
- This is used for urinary tract infections, acute exacerbations of chronic bronchitis, shigellosis, and traveler's diarrhea
For PCP Prophylaxis
- 1 DS tablet daily is the recommended prophylactic dose in HIV-infected adults 2, 1
- Alternatively, 1 DS tablet three times weekly is also acceptable 2
For PCP Treatment (True "Double Dose" Scenario)
When treating active Pneumocystis jirovecii pneumonia, significantly higher doses are required:
- Treatment dose: 75-100 mg/kg/day of sulfamethoxazole component (15-20 mg/kg/day trimethoprim) divided every 6 hours 1
- For an 80 kg adult, this translates to 2.5 DS tablets every 6 hours (or 5 DS tablets per day total)
- This represents approximately 5-6 times the standard prophylactic dose
Single-Dose Therapy Context
In older literature for uncomplicated UTIs in women, "double dose" sometimes referred to 2 DS tablets given as a single dose (total: 1600 mg sulfamethoxazole + 320 mg trimethoprim) 3, 4. However, this single-dose approach is less commonly used today.
Clinical Pitfall to Avoid
Do not confuse a "double-strength tablet" with "doubling the dose." When a provider orders "1 DS tablet," they are prescribing the standard formulation, not asking you to double anything. If higher doses are needed (as in PCP treatment), the prescription should specify the exact number of tablets and frequency (e.g., "2 DS tablets every 6 hours").