Half-Life and Washout Time for Single-Strength Bactrim
After a single dose of single-strength Bactrim (sulfamethoxazole 400 mg/trimethoprim 80 mg), the mean serum half-lives are approximately 10 hours for sulfamethoxazole and 8-10 hours for trimethoprim, with detectable drug levels present in blood for 24 hours post-administration 1.
Pharmacokinetic Details
Half-Life Values
The FDA-approved drug label provides the definitive pharmacokinetic parameters 1:
- Sulfamethoxazole: Mean serum half-life of 10 hours
- Trimethoprim: Mean serum half-life of 8-10 hours
Peak blood levels occur 1-4 hours after oral administration for both components 1.
Washout Period
Detectable amounts of both sulfamethoxazole and trimethoprim remain in the blood 24 hours after a single dose 1. However, this represents the minimum detection threshold rather than complete elimination.
For practical washout calculations:
- Using the standard 5 half-lives rule for >97% drug elimination:
- Sulfamethoxazole: Approximately 50 hours (10 hours × 5)
- Trimethoprim: Approximately 40-50 hours (8-10 hours × 5)
Excretion Characteristics
Both drugs are primarily eliminated renally 1:
- 84.5% of total sulfonamide recovered in urine (0-72 hours)
- 66.8% of free trimethoprim recovered in urine (0-72 hours)
- 30% of sulfamethoxazole excreted unchanged; remainder as N4-acetylated metabolite
Important Clinical Caveats
Renal Impairment
Patients with severely impaired renal function exhibit significantly increased half-lives of both components 1. This substantially prolongs the washout period and requires dosage adjustment during treatment.
Protein Binding
- Sulfamethoxazole: ~70% protein-bound
- Trimethoprim: ~44% protein-bound 1
The free (unbound) forms are therapeutically active, which is relevant for drug interactions and clinical effects.
Age-Related Variations
Research data shows variability in elimination: