[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Antiseizure medicines](https://durgical.com/en/pharmacology/classes/antiseizure-medicines/) › Phenytoin [Pharmacology](https://durgical.com/en/pharmacology/) · [Antiseizure medicines](https://durgical.com/en/pharmacology/classes/antiseizure-medicines/) # Phenytoin An old, inexpensive sodium channel blocker for seizures. Its levels rise steeply with small dose changes, and long use can enlarge the gums. Updated 2026-11-01 - Class: [Antiseizure medicines](https://durgical.com/en/pharmacology/classes/antiseizure-medicines/) - Ion channel: Voltage-gated sodium channel (Nav) - ATC code: N03AB02 - WHO Model List of Essential Medicines Also called: Diphenylhydantoin ## Mechanism of action Phenytoin blocks voltage-gated sodium channels in a voltage-dependent way, slowing their recovery from inactivation. This stops the sustained high-frequency discharges of a seizure focus while leaving normal, slower firing mostly intact. - Binds sodium channels during repeated depolarization - Slows the channel's recovery from inactivation - Sustained high-frequency firing stops - The seizure does not spread ## Pharmacokinetics **Absorption** Absorbed by mouth; enteral feeding preparations can lower levels **Distribution** Highly bound to albumin; in low albumin or kidney failure the free level is the useful one **Metabolism** In the liver by CYP2C9 and CYP2C19; metabolism saturates, so small dose increases can raise levels a lot (non-linear kinetics) **Half-life** On average about 14 hours with chewable tablets (range about 7 to 29 hours), longer at high levels ## Indications - Focal seizures and generalized tonic-clonic seizures [Epilepsy and seizures →](https://durgical.com/en/conditions/epilepsy-and-seizures/) - Prevention and treatment of seizures during or after neurosurgery - Status epilepticus (intravenous phenytoin or its prodrug fosphenytoin) [Epilepsy and seizures →](https://durgical.com/en/conditions/epilepsy-and-seizures/) ## Adverse effects ### Common - Nystagmus, unsteadiness (ataxia), slurred speech and drowsiness, which rise with the level - Gingival hyperplasia - Coarsening of facial features and excess hair - Rash ### Serious (rare, important) - Stevens-Johnson syndrome and toxic epidermal necrolysis (possible HLA-B*1502 link) - DRESS (multi-organ hypersensitivity) - Liver injury - Megaloblastic anaemia and loss of bone density (osteomalacia) - Fetal hydantoin syndrome ## Contraindications - Hypersensitivity to phenytoin or other hydantoins - Previous liver injury caused by phenytoin - Use with delavirdine ## Key interactions **Valproate and other highly protein-bound drugs** Displace phenytoin and change the free level **CYP2C9 or CYP2C19 inhibitors** Phenytoin levels rise quickly because metabolism is saturable **Hormonal contraceptives, anticoagulants and many other drugs** Phenytoin induces their metabolism and lowers their levels **Enteral feeding** Lower phenytoin levels ## What is monitored - Serum phenytoin level, free level if albumin is low - Signs of toxicity: nystagmus, ataxia, confusion - Complete blood count and liver tests - Gum health and bone health in long use ## Administration and safety - Oral forms (capsules, chewable tablets, suspension) are not interchangeable without care - Phenytoin dissolves poorly in water, which limits its intravenous use; the water-soluble prodrug fosphenytoin was developed for parenteral use - Withdrawn gradually: stopping suddenly can trigger status epilepticus ## Nursing care - Check levels and look for nystagmus, unsteadiness or confusion - Teach careful oral hygiene and regular dental visits - Teach to report rash, fever or swollen glands - Separate from enteral feeds as the protocol says and watch levels ## Pregnancy and breastfeeding Linked to malformations such as oral clefts and heart defects and to the fetal hydantoin syndrome; benefits and risks are weighed with specialist care. ## Memory aid Phenytoin: the Gums grow, the Hair grows, the eyes jerk (Nystagmus) and the kinetics go Non-linear. ## Test yourself Think of the answer, then tap the question to see it. ## Why can a small dose increase of phenytoin cause toxicity? Its liver metabolism saturates at therapeutic levels, so kinetics become non-linear and levels rise much more than the dose. ## Which cosmetic adverse effect is typical of long-term phenytoin? Gingival hyperplasia; coarsening of facial features and excess hair also occur. ## Which early neurological sign suggests rising phenytoin levels? Nystagmus, followed by unsteadiness (ataxia), slurred speech and confusion. ## Why is the free phenytoin level preferred in a patient with low albumin? Phenytoin is highly bound to albumin; with low albumin the free, active fraction is higher than the total level suggests. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/pharmacology/phenytoin/) ## Related drugs - [Fosphenytoin](https://durgical.com/en/pharmacology/fosphenytoin/) - [Carbamazepine](https://durgical.com/en/pharmacology/carbamazepine/) ## Related conditions - [Epilepsy and seizures](https://durgical.com/en/conditions/epilepsy-and-seizures/) [← Back to the class: Antiseizure medicines](https://durgical.com/en/pharmacology/classes/antiseizure-medicines/) Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given. Sources: [Phenytoin: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a682022.html) (MedlinePlus (U.S. National Library of Medicine), 2023); [DailyMed - PHENYTOIN tablet, chewable](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=78b38e7b-1460-4adc-b738-320ffa6259c2) (DailyMed (U.S. National Library of Medicine), 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 20 (McGraw Hill, 2023); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/) (World Health Organization, 2025). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/pharmacology/phenytoin/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/phenytoin/