Phenytoin
Class: anticonvulsant – hydantoin
Indications (NB some may be unlicensed): epilepsy, prophylaxis in neurosurgery, arrhythmias
Contraindications/cautions: low albumin, diabetes, elderly (reduced clearance)
Adverse reactions: common: gingival hyperplasia; less common: slurred speech, confusion, dizziness, blood disorders, skin reactions, hepatitis
Metabolism/clearance: metabolised by metabolising enzyme CYP2C8/9 mainly in the liver
Interactions:
- increased clinical effect/toxicity of phenytoin (due to increased blood concentrations) may occur with some CYP metabolising enzyme inhibitors (see above) e.g. fluconazole, ketoconazole, voriconazole
- decreased clinical effect/toxicity of phenytoin (due to decreased blood concentrations) may occur with some CYP metabolism enzyme inducers (see above) e.g. phenobarbitone, rifampicin
- decreased clinical effect/toxicity of some drugs (due to decreased blood concentrations of them) may occur with phenytoin due to metabolising enzyme induction by phenytoin e.g. aprepitant, buspirone, amitriptyline, carbamazepine, clonazepam, dexamethasone, diazepam, domperidone, fentanyl, itraconazole, ketoconazole, methadone, midazolam, NSAIDs (e.g. diclofenac), olanzapine, ondansetron, phenytoin, prednisone, quetiapine, triazolam, warfarin
| Dosing: | |
|---|---|
| oral: | 100 to 300 mg/24 hours (titrate to plasma concentrations) |
| subcut: | inj available but not given subcut |
| rectal: | not available |
Syringe driver: not applicable
Mechanism of action: inhibits spread of seizure through the motor cortex possibly via sodium channels
Peak response: 7 to 10 days (if loaded 8 to 12 hours)
Notes:
- monitor plasma concentrations
- small dose increases may result in large plasma concentration increases
- if the patient has NG feeds these will affect phenytoin concentrations