Which statement correctly identifies an interaction that requires adjustment when combining chemotherapy with another drug?

• A. Acetaminophen with methotrexate requires no adjustment.
• B. Grapefruit juice with any chemotherapy has no effect.
• C. Allopurinol with 6-mercaptopurine requires avoidance or dose adjustment.
• D. Vitamin C with cyclophosphamide always improves efficacy.

Answer: (C)

Drug interactions can change how a chemotherapy drug is metabolized or cleared, altering both its effectiveness and risk of toxicity. The classic example is combining allopurinol with 6-mercaptopurine. Allopurinol inhibits xanthine oxidase, the enzyme that helps break down 6-mercaptopurine. When this enzyme is blocked, 6-mercaptopurine levels rise, leading to greater bone marrow suppression and other toxicities. Because of this, you either avoid the combination or significantly reduce the 6-mercaptopurine dose and monitor closely.

The other statements aren’t reliable: grapefruit juice can affect the metabolism of some chemo agents by inhibiting enzymes like CYP3A4, so saying it has no effect isn’t accurate. Vitamin C with cyclophosphamide and acetaminophen with methotrexate can influence toxicity or efficacy depending on dose and regimen, so blanket claims that they always foolproof or require no adjustment aren’t correct.