The
serum levels of
quinidine and its therapeutic effects can be markedly reduced by
rifampicin. In one case
quinidine levels were reduced from 4 to 0.5 micrograms/mL.
The dosage of
quinidine will need to be increased (possibly more than doubled in some patients) if
rifampicin is given concurrently. The
quinidine dose will need to be reduced when the
rifampicin is stopped. Monitor the
serum levels where possible.