Which of the following is NOT an inhibitor of cytochrome P450 enzymes?

• A. Cimetidine
• B. Ketoconazole
• C. Erythromycin
• D. Ibuprofen

Answer: (D)

Understanding how certain drugs affect liver enzymes helps explain why some medicines interact. Cytochrome P450 enzymes metabolize many drugs, so inhibitors raise levels of medications that rely on those enzymes, potentially causing toxicity or exaggerated effects.

Ibuprofen does not act as a meaningful inhibitor of the cytochrome P450 system. In contrast, cimetidine can modestly inhibit several CYP enzymes, which can slow the metabolism of some drugs. Ketoconazole is a strong inhibitor of CYP3A4, dramatically increasing levels of drugs processed by that enzyme. Erythromycin also inhibits CYP3A4, similarly elevating the levels of coadministered drugs. Therefore, ibuprofen is the one that does not inhibit cytochrome P450 enzymes.