Phenytoin Sodium & Fosphenytoin Sodium
Hydantoin Antiepileptic (Class Ib Voltage-Gated Na+ Channel Blocker) • Schedule H (Drugs Rules, 1945 — High-Alert Narrow Therapeutic Index Drug)
Level 1 — Quick Professional Summary (At a Glance)
Phenytoin Sodium & Fosphenytoin Sodium
Monograph Code: [11.13] | Stable ID: TIP-MED-000217
Eptoin (Abbott), Dilantin (Pfizer)
Hydantoin Antiepileptic (Class Ib Voltage-Gated Na+ Channel Blocker)
Neurology & Antiepileptic Medicines (Ch 11)
Oral Tablets / Capsules: 50 mg, 100 mg, 300 mg ER; Oral Suspension: 30 mg/5 mL, 125 mg/5 mL; IV Injection: 50 mg/mL (2 mL ampoule = 100 mg).; Oral Tablets / Capsules: 50 mg, 100 mg, 300 mg ER; Oral Suspension: 30 mg/5 mL, 125 mg/5 mL; IV Injection: 50 mg/mL (2 mL ampoule = 100 mg).
Route: Oral / Parenteral / Topical as specified by formulation
9–10. Mechanism of Action & Pharmacodynamics
Stabilises neuronal membranes in the motor cortex by prolonging the voltage-dependent inactivation state of fast neuronal sodium channels (Nav), suppressing high-frequency repetitive action potential firing without causing general CNS depression.
12. Therapeutic Indications
- Generalised tonic-clonic seizures and focal (partial) seizures
- Status epilepticus (second-line IV loading after benzodiazepine administration) and post-neurosurgical seizure prophylaxis
13 & 16. Contraindications, Precautions & Warnings
- Sinus bradycardia, sinoatrial block, second- or third-degree AV block, or Adams-Stokes syndrome (IV formulation)
- Co-administration with Delavirdine or Rilpivirine (potent CYP3A4/P-gp induction causes virological failure)
- Absence (petit mal) or myoclonic seizures (may exacerbate seizures)
Saturable hepatic CYP2C9/2C19 metabolism: reduce maintenance dose in hepatic impairment and monitor rigorously for cerebellar toxicity (nystagmus, ataxia, dysarthria).
14. Dosage & Administration Context (Educational Reference Summary)
Administration & Timing: Take consistently with food or milk to reduce gastric irritation; separate by >= 2 hours from antacids, calcium, iron, or nasogastric enteral feeds. IV infusion must NEVER exceed 50 mg/min in adults (25 mg/min in elderly) and must be diluted ONLY in 0.9% Normal Saline (precipitates in 5% Dextrose!).
Level 2 — Detailed Pharmacology, PK, Interactions & Special Populations
11 & 19. Pharmacokinetics (ADME), Renal & Hepatic Considerations
Pharmacokinetic Profile: Absorption, distribution, metabolism, and excretion follow class pharmacokinetic profile; verify patient renal/hepatic function.
Renal Considerations: In severe renal impairment (CrCl < 25 mL/min) or hypoalbuminaemia (< 3.2 g/dL), plasma protein binding decreases and the free (active) phenytoin fraction increases; apply the Sheiner-Tozer albumin/uraemia correction equation or monitor free phenytoin levels.
Hepatic Considerations: Saturable hepatic CYP2C9/2C19 metabolism: reduce maintenance dose in hepatic impairment and monitor rigorously for cerebellar toxicity (nystagmus, ataxia, dysarthria).
15 & 17. Adverse Effects & Clinically Significant Drug Interactions
- Concentration-dependent CNS toxicity: Nystagmus (>20 mcg/mL), Ataxia (>30 mcg/mL), Dysarthria & Lethargy (>40 mcg/mL)
- Chronic effects: Gingival hyperplasia (20–40%), hirsutism, coarsening of facial features, Vitamin D deficiency osteomalacia, folate-deficiency megaloblastic anaemia
- Idiosyncratic: SJS/TEN (strongly associated with HLA-B*15:02 allele in South Asian populations) and DRESS syndrome; IV extravasation causes 'Purple Glove Syndrome'
- Potent CYP3A4/2C9/2C19 & UGT Inducer: Markedly reduces plasma efficacy of Oral Contraceptives, DOACs (Apixaban/Rivaroxaban), Warfarin, Carbamazepine, Lamotrigine, and Corticosteroids.
- CYP2C9/2C19 Inhibitors (Isoniazid, Fluconazole, Voriconazole, Amiodarone, Valproate): Precipitate acute Phenytoin toxicity due to saturated zero-order clearance.
- Enteral Tube Feeds & Calcium/Antacid Supplements: Chelate oral Phenytoin and reduce bioavailability by up to 70%; hold enteral feeds 2 hours before and 2 hours after oral Phenytoin suspension/tablets.
18, 20, 21 & 22. Special Populations, Monitoring, Counselling & Storage
Pregnancy, Lactation, Paediatric & Geriatric Use: Evaluate maternal-foetal benefit-risk ratio and age-appropriate organ function before prescribing.
Clinical & Laboratory Monitoring: Monitor clinical response, baseline renal/hepatic parameters, and relevant serum biomarkers.
Patient Counselling Points:- Never switch between different manufacturer brands or between Phenytoin Sodium (92% phenytoin acid) and Phenytoin Free Acid (100% phenytoin) without physician review and TDM.
- Report unsteady gait, slurred speech, double vision, or any new skin rash/mouth ulcers immediately.
- Maintain strict dental hygiene (soft toothbrush and flossing) to minimise gum overgrowth, and separate from calcium/antacids by at least 2 hours.
Storage & Dispensing Governance: Store tightly closed below 30°C protected from moisture. High-Alert NTI check: verify 100 mg tablet/capsule vs 30 mg paediatric tablet vs 25 mg/5 mL suspension.
Visual Pharmacology Mechanism Diagram (PLATE-8.6)
23. Traceable Source References & Jurisdiction
Primary Reference Record: Indian Pharmacist Desk Reference — Professional Edition 2.7 (2026–27) by THE INDIAN PHARMACIST Editorial Reference; NFI 7th Ed (2026); Register D.8 p. 254.
Authoritative Verification Portals: Indian Pharmacopoeia Commission (IPC) • CDSCO (India Jurisdiction) • NCBI Bookshelf • DailyMed (US FDA Label Reference)
Last Checked Date: 2026-11-08 • Related Study Guide: Epilepsy & Seizure Disorders ASM Guide →