Enzyme induction by rifampicin is not immediate. It develops over about a week or more as new enzyme is synthesised, and it persists for roughly two weeks after the last dose while that enzyme is cleared.
That timing drives two errors. Starting rifampicin can make a stable drug gradually ineffective over the first fortnight; stopping it can let the same drug rise to toxic levels over the following weeks if its dose was increased. Plan the interacting drug's dose and monitoring across both transitions, not only on the day rifampicin changes.
- Expect induction to build over the first one to two weeks of rifampicin.
- Expect it to wear off over about two weeks after rifampicin stops.
- Monitor levels or effect of interacting drugs through both transitions.
- Reduce any dose that was raised for induction once rifampicin has stopped.
Why it matters
The dangerous moment is often when rifampicin stops, not when it starts.
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