Which statement is least likely to affect INR control in a patient taking warfarin?

Which statement is least likely to affect INR control in a patient taking warfarin?

• A. Drinking grapefruit juice every morning
• B. Drinking orange juice
• C. Eating grapefruit segments
• D. Taking vitamin K-rich foods

Answer: (A)

Warfarin INR control hinges mainly on vitamin K intake and, to a lesser but real extent, on factors that alter warfarin’s metabolism. The strongest and most predictable influence comes from consuming foods rich in vitamin K, which counteracts warfarin and lowers INR; thus vitamin K–rich foods clearly affect INR and require consistent intake. Grapefruit juice can affect drug metabolism for many medications, but warfarin’s primary breakdown is by CYP2C9, and the impact of grapefruit on this pathway is not reliably large or consistent across individuals. The same goes for orange juice—it's not a major, predictable modifier of warfarin metabolism or INR. Therefore, among the options, grapefruit juice is least likely to produce a reliable change in INR control, while vitamin K–rich foods have a clear, direct effect.

Warfarin INR control hinges mainly on vitamin K intake and, to a lesser but real extent, on factors that alter warfarin’s metabolism. The strongest and most predictable influence comes from consuming foods rich in vitamin K, which counteracts warfarin and lowers INR; thus vitamin K–rich foods clearly affect INR and require consistent intake.

Grapefruit juice can affect drug metabolism for many medications, but warfarin’s primary breakdown is by CYP2C9, and the impact of grapefruit on this pathway is not reliably large or consistent across individuals. The same goes for orange juice—it's not a major, predictable modifier of warfarin metabolism or INR. Therefore, among the options, grapefruit juice is least likely to produce a reliable change in INR control, while vitamin K–rich foods have a clear, direct effect.