Blood serum levels and half-life times of Rifampicin (RAMP) have been investigated after the first administration, at various time intervals during long-term treatment and following a test dose given 30 days after the end of long-term treatment in tuberculous patients. After a marked drop of the values at the 6th and 12th hour following administration, which takes place within the first 30 days of treatment together with a sharp decrease in the half-life times, no further appreciable changes have been observed up to the 10th month of treatment. After a test dose given 30 days after the end of treatment, a curve comparable to that observed following the first administration is obtained. The changes are likely to depend upon an increased metabolization of the drug to its desacetyl-derivative which is more rapidly excreted and very poorly absorbed. This behaviour does not affect to any appreciable extent the therapeutic effectiveness of RAMP.

Patients with normal liver function present a dose-related change in the kinetics of rifampicin. The drug interferes with serum bilirubin levels. Interference with serum bilirubin levels practically disappears during repeated administration. This fact is paralleled by a change in the slope of the excretory part of rifampicin blood serum level curve. Patients with liver cirrhosis or with other type of severe liver impairment present higher blood serum levels of the antibiotic. The slope of the curve is flat and remains practically constant during repeated administration. The interference with blood serum bilirubin remains unchanged throughout the treatment period. In such patients, a dose of 6–8 mg/kg b.w. of rifampicin should not be exceeded.

Dose-related changes of the overall elimination rate constant were observed both after single and repeated administration of rifampicin. The threshold dose appears to be between 12 and 16 mg/kg in patients with normal liver function. Interference between rifampicin and bilirubin is confirmed. It does not appear dose-related and decreases during repeated administration.

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