A drug's net effect on another can change with dose. When a drug both inhibits one enzyme and induces others, inhibition may dominate at low exposure while induction takes over as the dose rises. Induction also takes one to two weeks to develop and to wear off, whereas inhibition is usually immediate.
So the effect of an interacting drug may reverse during titration, and a level measured early may not predict the level at steady state.
- Recheck levels of narrow-therapeutic-index drugs after each step in titrating an interacting drug.
- Allow one to two weeks for induction to appear or resolve before judging a level.
- Expect inhibition to act within days and to resolve quickly on stopping.
- Document the interacting drug's dose alongside any level you record.
Why it matters
A single interaction label rarely captures how the effect changes over dose and time.
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