Rifampicin induces CYP3A4, other CYP enzymes and P-glycoprotein. Induction builds over roughly one to two weeks and wears off over a similar period after the last dose, because it depends on synthesis and turnover of new enzyme.
So a drug whose dose was raised to overcome induction can reach toxic levels in the weeks after rifampicin stops, and a drug affected by it needs cover beyond the end of the course.
- Check every co-prescribed drug for CYP3A4 or P-glycoprotein metabolism before starting rifampicin.
- Plan dose reductions for induced drugs, such as tacrolimus or warfarin, over the two weeks after rifampicin stops.
- Warn women using hormonal contraception that it may fail during and for weeks after rifampicin.
- Monitor levels or effect, such as INR, closely at both the start and the end of a rifampicin course.
Why it matters
Most interaction harms from rifampicin happen at the transitions, not in the steady middle of a course.
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