This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Phenobarbitone and warfarin interaction results in: March 2005 
Select and explain. 
A. Increased absorption of warfarin, B. Increased metabolism of warfarin, C. Displacement of warfarin from the binding site, D. All of the above
B: Increased metabolism of warfarin
Answer: B: Increased metabolism of warfarin 
Explanation: Ans. B: Increased metabolism of warfarin Barbiturates increase the metabolism of oral anticoagulants, resulting in reduced anticoagulation effects. If co-administration is unavoidable, monitoring of INR and adjustment of anticoagulant dose is needed. An increase in anticoagulant dose (approximately 30-60%) may be needed when adding a barbiturate. Withdrawal of barbiturates will result in reduced anticoagulant requirement over several weeks; a reduction in the anticoagulant dose may be needed to prevent bleeding complications