Which of the following is NOT a CYP inhibitor?

• A. Grapefruit
• B. St. John's wort
• C. Amiodarone
• D. Phenobarbital

Answer: (D)

Phenobarbital is not classified as a CYP inhibitor; in fact, it acts as an inducer of certain cytochrome P450 enzymes, particularly CYP2C9 and CYP3A4. By inducing these enzymes, phenobarbital can accelerate the metabolism of drugs that are substrates for these pathways, leading to reduced effectiveness of those medications. This is important in clinical practice because it can affect dosing and therapeutic outcomes when administering other medications alongside phenobarbital.

In contrast, grapefruit juice is well known for its ability to inhibit CYP3A4, leading to increased blood levels of many drugs metabolized by this enzyme. St. John's wort is also recognized as a CYP inducer, particularly affecting CYP3A4, and can lead to decreased plasma concentrations of drugs that rely on this metabolic pathway. Amiodarone is another substance recognized for its inhibitory effect on various CYP enzymes, contributing to potential drug interactions. Therefore, the correct answer highlights phenobarbital's role as an inducer rather than an inhibitor, distinguishing it clearly from the other choices listed.