Rifampicin reduces the AUCs of
trimethoprim and sulfamethoxazole by 56% and 28%, respectively, in HIV-positive subjects, but apparently has no effect on
trimethoprim in healthy subjects.
Co-trimoxazole can increase
rifampicin serum levels in patients with
tuberculosis by almost a third.
The
efficacy of
co-trimoxazole prophylaxis may be reduced in HIV-positive patients taking
rifampicin. Bear this in mind when using the combination. The risk of hepatotoxicity may be increased due to raised
rifampicin levels. Liver function tests should already be closely monitored so no additional precautions seem necessary.