Which antiepileptic drug is known for having 'nonlinear' or 'zero-order' pharmacokinetics at therapeutic concentrations?
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Correct answer: Phenytoin
At low doses, Phenytoin elimination is first-order, but as plasma levels reach the therapeutic range, hepatic enzymes become saturated. This transition to zero-order kinetics means that small dose increases can lead to disproportionately large and potentially toxic increases in plasma concentration.
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