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Pharmacokinetics of Phenytoin 1

Pharmacokinetics of phenytoin



A 24-year-old woman with a history of epilepsy is seen in the clinic for a follow-up visit. She reports that she has been experiencing episodes of confusion, dizziness, and abnormal facial movements. She has been taking her medication as prescribed. She is currently on phenytoin for seizure control. On physical examination, her vital signs are within normal limits, but you notice horizontal nystagmus and mild ataxia. Which of the following best describes the pharmacokinetic property of the drug that contributes to the patient's current symptoms?


A. Zero-order kinetics

B. First-order kinetics

C. Second-order kinetics

D. Non-competitive inhibition

E. Competitive inhibition


A. Zero-order kinetics


Phenytoin is an antiepileptic drug that follows zero-order kinetics at therapeutic or higher doses, meaning that a constant amount of drug is eliminated per unit of time, regardless of the total drug concentration in the body. This is because the enzymes responsible for metabolizing phenytoin become saturated at therapeutic doses, and any increase in dose can lead to disproportionately high levels of the drug in the body, resulting in toxicity. The clinical features of phenytoin toxicity include nystagmus, ataxia, and confusion, which this patient is experiencing. The other choices listed do not accurately describe the pharmacokinetics of phenytoin. First-order kinetics, where a constant fraction of drug is eliminated per unit of time, is the most common type of metabolism for drugs, but phenytoin is a notable exception. Second-order kinetics is not a recognized type of drug metabolism. Non-competitive and competitive inhibition refer to enzyme inhibition, which is not relevant in this case.


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