When a previously effective drug seems to fail, check for a newly started enzyme inducer. Rifampicin, carbamazepine, phenytoin, phenobarbital and St John's wort can lower exposure to CYP3A4 substrates dramatically. The effect builds over about a week and persists for a week or two after the inducer is stopped, so the timing of the problem may not match the start date.
- Take a full medicines history, including herbal products, when a drug loses effect.
- Remember induction develops and resolves over days to weeks.
- Re-check doses of interacting drugs when an inducer is stopped.
Why it matters
Induction interactions are often missed because their timing does not line up neatly with a prescription change.
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