Inducers such as rifampicin, carbamazepine, phenytoin and phenobarbital increase CYP enzyme production. The effect builds over one to two weeks and, after the inducer is stopped, fades over a similar period as new enzyme is gradually cleared.
The danger is at the transitions. Starting an inducer can drop levels of warfarin, tacrolimus, hormonal contraceptives or antiretrovirals; stopping it can let those levels climb toxic over the following weeks if doses were raised to compensate. Plan dose changes and monitoring for both directions.
- Expect induction to build over one to two weeks after starting an inducer
- Expect it to fade over about two weeks after stopping
- Recheck levels or INR after both starting and stopping an inducer
- Reduce doses that were raised to compensate once the inducer is stopped
Why it matters
Toxicity after stopping an inducer is the half of the interaction most often missed.
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