A patient of DVT was stabilized on warfarin. This person was given phenobarbitone. Which of the following is the likely interaction between the two drugs?
**Core Concept**
Warfarin is a vitamin K antagonist used for anticoagulation, and its efficacy is dependent on the cytochrome P450 enzyme system, particularly the CYP2C9 enzyme. Phenobarbitone, a barbiturate, is a known inducer of the cytochrome P450 enzymes.
**Why the Correct Answer is Right**
Phenobarbitone induces the CYP2C9 enzyme, which is responsible for the metabolism of warfarin. By inducing this enzyme, phenobarbitone increases the metabolic rate of warfarin, resulting in decreased warfarin plasma concentrations. This interaction can lead to a decrease in the anticoagulant effect of warfarin, potentially increasing the risk of thromboembolic events.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the interaction between warfarin and phenobarbitone. The interaction between warfarin and phenytoin is a separate topic.
* **Option B:** This option is incorrect because phenobarbitone is an inducer, not an inhibitor, of the cytochrome P450 enzymes.
* **Option C:** This option is incorrect because the interaction between warfarin and phenobarbitone is not related to warfarin's effect on platelet aggregation.
**Clinical Pearl / High-Yield Fact**
When a patient is stabilized on warfarin and is prescribed a drug that induces the cytochrome P450 enzymes, such as phenobarbitone, the patient's INR (International Normalized Ratio) may decrease, and the anticoagulant effect of warfarin may be reduced.
**Correct Answer:** C.