What is a common pharmacokinetic property of phenytoin at high doses?

Phenytoin demonstrates zero-order kinetics at high doses due to its metabolism being saturable. In the context of pharmacokinetics, zero-order kinetics means that the rate of drug elimination is constant, regardless of the drug concentration in the bloodstream. This occurs because the enzymes responsible for metabolizing phenytoin become saturated when high doses are administered, leading to a situation where the body cannot process the drug any faster, causing the plasma concentration to increase disproportionately with each additional dose.

In contrast, at normal therapeutic doses, phenytoin may exhibit first-order kinetics, where the elimination rate is proportional to the drug concentration. However, as the dose rises to levels that exceed the metabolic capacity of the liver enzymes, the zero-order kinetics becomes predominant. This is a critical concept in understanding how dose adjustment and monitoring are vital for medications with saturable metabolism like phenytoin, especially to avoid toxicity.

Other options like linear absorption and rapid metabolism do not apply correctly to this scenario. Linear absorption suggests that the absorption rate remains constant and is independent of concentration, which does not occur with phenytoin in the case of its saturable metabolism. Rapid metabolism would imply that the drug is processed quickly, which contradicts the zero-order