## Phenytoin: Pharmacokinetics and Clinical Properties ### Correct Statements **High-Yield:** Phenytoin exhibits zero-order (saturation) kinetics at therapeutic doses. This means that the elimination rate does not increase proportionally with dose, making small dose increases potentially cause large changes in plasma concentration. This is a critical clinical feature [cite:KD Tripathi 8e Ch 12]. **Key Point:** Phenytoin is 90% protein-bound to albumin. Drug interactions occur through competitive displacement from protein binding sites, increasing the free (active) fraction of phenytoin or other displaced drugs [cite:KD Tripathi 8e Ch 12]. **Key Point:** Phenytoin is a potent inducer of hepatic cytochrome P450 enzymes (CYP2C9, CYP2C19, CYP3A4). This increases the metabolism of many drugs, including oral contraceptives, reducing their efficacy [cite:Harrison 21e Ch 369]. ### The Incorrect Statement **Clinical Pearl:** Gingival hyperplasia caused by phenytoin is NOT fully reversible. While it may improve after discontinuation, it often does NOT resolve completely. The hyperplastic tissue may require surgical intervention (gingivectomy) for complete resolution [cite:KD Tripathi 8e Ch 12]. This is a permanent or semi-permanent adverse effect, not a reversible one. ### Phenytoin Adverse Effects Summary | Adverse Effect | Reversibility | Management | | --- | --- | --- | | Gingival hyperplasia | Partial/Irreversible | Gingivectomy, switch drug | | Hirsutism | Reversible | Discontinue or switch | | Coarse facial features | Irreversible | Switch drug | | Ataxia, nystagmus | Reversible | Dose reduction | | Fetal hydantoin syndrome | N/A (teratogenic) | Avoid in pregnancy | | Megaloblastic anemia | Reversible | Folic acid supplementation | **Mnemonic:** PHENYTOIN adverse effects — **PHE**nytoin causes **N**ystagmus, **Y**ellow teeth, **T**oxicity, **O**ver-growth (gingival), **I**nduction (enzyme), **N**europathy (peripheral).
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.