Pharmacokinetics of trimethoprim given in single daily doses for three days.

Trimethoprim in a single daily dose of 300 mg was administered at night to 8 healthy volunteers for 3 days. Serum concentrations of trimethoprim were measured after 12, 36, 60 and 84 h. The average trimethoprim concentrations were 3.0, 4.0, 4.7 and 0.96 micrograms/ml, respectively. The urinary concentrations were in excess of the minimum inhibitory concentration for most urinary pathogens for up to 5 days after the start of oral medication. The individual variation in bioavailability and urinary excretion was reflected in the varying amount of unchanged trimethoprim excreted in the urine, between 66 and 95%.