**Question:** Phenobarbitone and warfarin interaction results in: March 2005

**Core Concept:**
The interaction between phenobarbitone and warfarin is of clinical significance due to the increased risk of bleeding. Phenobarbitone is a barbiturate, a group of sedative-hypnotic drugs used for seizure disorders, whereas warfarin is an anticoagulant, a medication used to prevent blood clots. Understanding this interaction is crucial for safe prescribing and monitoring of these medications.

**Why the Correct Answer is Right:**

B. Phenobarbitone increases the anticoagulant effect of warfarin.

When phenobarbitone is administered along with warfarin, it increases the anticoagulant effect of warfarin. This is because phenobarbitone is a potent inhibitor of liver microsomal enzymes responsible for the clearance of warfarin. As a result, warfarin levels increase in the blood, leading to an increased anticoagulant effect.

**Why Each Wrong Option is Incorrect:**

A. Warfarin increases the sedative effect of phenobarbitone:
- Warfarin does not directly affect the sedative effect of phenobarbitone as the anticoagulant effect of warfarin is primarily due to its inhibition of vitamin K-dependent clotting factors, while phenobarbitone's sedative effect is due to its interaction with gamma-aminobutyric acid (GABA) receptors in the central nervous system.

C. Phenobarbitone increases the anticoagulant effect of warfarin:
- As mentioned earlier, phenobarbitone inhibits liver microsomal enzymes responsible for warfarin clearance, leading to increased warfarin levels and an increased anticoagulant effect. However, the correct answer is option B, as it explicitly states the interaction between the two medications.

D. No interaction between phenobarbitone and warfarin:
- As explained above, phenobarbitone and warfarin have a significant interaction, which increases the anticoagulant effect of warfarin.

**Clinical Pearl:**

When prescribing these medications together, careful monitoring of INR (International Normalized Ratio) is essential to prevent excessive anticoagulation, which can lead to severe bleeding complications. The INR should be monitored more frequently, and the warfarin dose may need to be adjusted based on the INR results. This is crucial for ensuring patient safety and preventing adverse events.