Phenytoin's effect on other drugs is mediated through which system?

• A. Hepatic microsomal cytochrome P450 enzymes
• B. Renal clearance
• C. Pulmonary metabolism
• D. Lymphatic diffusion

Answer: (A)

The main concept is that phenytoin interacts with other drugs by inducing the liver’s enzyme system that metabolizes most drugs. Phenytoin strongly activates hepatic microsomal cytochrome P450 enzymes, often through nuclear receptors that boost the production of these enzymes. This induction speeds up the metabolism of many coadministered medications, lowering their blood levels and reducing their effects (for example, oral contraceptives or warfarin may become less effective). Because of this, dose adjustments or alternative therapies are sometimes needed. This mechanism is about enzyme induction in the liver, not changes in renal clearance, pulmonary metabolism, or lymphatic diffusion. Phenytoin can also autoinduce its own metabolism over time, shortening its own duration of action.