Most interaction checking happens at the moment of prescribing, which is the wrong moment for half of the dangerous interactions. An enzyme inducer added to a stable regimen causes trouble on the way in; an inhibitor causes trouble on the way out.
Rifampicin started for tuberculosis induces CYP3A over one to two weeks, and the warfarin, tacrolimus, or oral contraceptive doses climb to compensate. The problem arrives when rifampicin stops, induction wanes over another one to two weeks, and the compensating dose is now a large overdose. The mirror image is a CYP3A inhibitor - clarithromycin, azole antifungals, ritonavir - where the dose was reduced and then never restored, or restored too fast.
So when an interacting drug is started, write down the date the compensating change was made, the date the interacting drug is due to stop, and who reviews the dose afterwards. The interaction has two ends and the second one has no prescription attached to prompt anybody.
- Record the planned stop date of the interacting drug in the same note as the dose change.
- Name who will review the dose after the interacting drug finishes.
- Expect induction and its resolution to take one to two weeks, not hours.
- Rifampicin is the commonest culprit in Indian practice, and its course is long.
- Book the monitoring test for after the interacting drug stops, at the time you start it.
Why it matters
The second half of an interaction has no prescription event attached to it, so nothing in the system prompts anyone to unwind the change.
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