Trimethoprim is well absorbed from the GI tract, and peak blood levels are achieved in about 2 hours. Tissue levels often exceed those of plasma, and the urine concentration of trimethoprim may be 100 times that of the plasma. Trimethoprim readily enters the CSF if inflammation is present. The half-life of the drug is approximately 11 hours. Sulfamethoxazole (t1/2 = 10 hours) is frequently coadministered with trimethoprim in a fixed dose ratio of 1:5 (trimethoprim to sulfamethoxazole).
Peak drug levels in plasma are achieved in 1 to 4 hours following oral administration and 1 to 1.5 hours after IV infusion. At this time, the TMP-SMX plasma ratio is 1:20, which is the ratio most effective for producing a synergistic effect against most susceptible pathogens. The ratio is also influenced by the greater lipid solubility of trimethoprim, which results in its larger volume of distribution. Both trimethoprim and sulfamethoxazole bind to plasma protein (45 and 66% respectively) and both are metabolized in the liver. Approximately 40 to 60% of both parent drugs and their metabolites is excreted by the kidney within 24 hours; in moderate to severe renal dysfunction the dose should be reduced by approximately one-half. Only the parent compounds are excreted in the bile. Both drugs cross the placenta and are found in breast milk (see adverse effects).
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