The half-life is 8 to 10 hours. Trimethoprim blood levels are dose and time dependent. Table I gives the mean serum concentrations as a function of time for both of these doses. Trimethoprim exists in the blood as free, protein bound and metabolized forms. The free form is considered to be the therapeutically active form.
Excretion is chiefly by the kidneys through glomerular filtration and tubular secretion.
Urine concentrations are considerably higher than are concentrations in the blood. Urine levels are time and dose related. Single doses of 200 mg produced urinary trimethoprim levels approximately twice as great. These levels are well above the MIC's required to be effective against the majority of urinary pathogens. Concentrations of trimethoprim in prostatic and vaginal secretions are consistently greater than those found in the serum.