1. Clinical & Scientific Overview
Phenytoin Sodium & Fosphenytoin Sodium (TIP-MED-000217, Reference Code [11.13]) is classified under
Neurology & Antiepileptic Medicines (Ch 11) as a Hydantoin Antiepileptic (Class Ib Voltage-Gated Na+ Channel Blocker).
Within the Indian regulatory and compendial framework, it is governed under Schedule H (Drugs Rules, 1945 — High-Alert Narrow Therapeutic Index Drug).
Eptoin (Abbott), Dilantin (Pfizer)
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).
3. 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.
4. Therapeutic Indications & Evidence-Supported Uses
- Generalised tonic-clonic seizures and focal (partial) seizures
- Status epilepticus (second-line IV loading after benzodiazepine administration) and post-neurosurgical seizure prophylaxis
5. Educational Dosage Context, Formulations & Administration
Educational Dosing & Regimen Summary: Educational Reference Summary: Typical adult maintenance dose is 300 mg/day orally (100 mg TID or 300 mg extended-release OD if validated). Because hepatic CYP2C9/2C19 hydroxylation saturates near therapeutic concentrations (Michaelis-Menten kinetics), dose increments above 300 mg/day must be small (25–30 mg) with Therapeutic Drug Monitoring (target total trough: 10–20 mcg/mL; free phenytoin: 1–2 mcg/mL).
Administration & Meal 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!).
6. Pharmacokinetics, Metabolism & Renal/Hepatic Adjustment
Absorption, distribution, metabolism, and excretion follow class pharmacokinetic profile; verify patient renal/hepatic function.
Saturable hepatic CYP2C9/2C19 metabolism: reduce maintenance dose in hepatic impairment and monitor rigorously for cerebellar toxicity (nystagmus, ataxia, dysarthria).
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.
Saturable hepatic CYP2C9/2C19 metabolism: reduce maintenance dose in hepatic impairment and monitor rigorously for cerebellar toxicity (nystagmus, ataxia, dysarthria).
7. Contraindications, Precautions & High-Risk 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)
Key Clinical Precautions: Saturable hepatic CYP2C9/2C19 metabolism: reduce maintenance dose in hepatic impairment and monitor rigorously for cerebellar toxicity (nystagmus, ataxia, dysarthria).
8. Adverse Effects Profile
- 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'
- Monitor for severe hypersensitivity, organ toxicity, or dose-limiting adverse events.
9. Major Drug–Drug, Drug–Food & Drug–Herb Interactions
- 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.
10. Clinical Monitoring, Patient Counselling & Storage
Clinical & Laboratory Monitoring: Monitor clinical response, baseline renal/hepatic parameters, and relevant serum biomarkers.
Pharmacist 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, Stability & Dispensing Safeguards: 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.
11. Visual Pharmacology & Mechanism Diagram (svg-v339-01-adme-12-step-journey)
12. Authoritative References, Regulatory Sources & Editorial Disclaimer
Primary Compendial & Guideline Reference: Indian Pharmacist Desk Reference — Professional Edition 2.7 (2026–27) by Tejas Nartam; NFI 7th Ed (2026); Register D.8 p. 254.
Official Verification Portals: Indian Pharmacopoeia Commission (IPC) & National Formulary of India (NFI) • Central Drugs Standard Control Organization (CDSCO) • Ministry of Health & Family Welfare — NLEM 2022 • NCBI Bookshelf / StatPearls Pharmacology Reference • DailyMed (US FDA Official Prescribing Reference)
Last Medically & Pharmaceutically Reviewed: 2026-11-10 • Verification Status: PENDING_CLINICAL_REVIEW
Educational & Clinical Disclaimer: This reference profile is published solely for pharmacy education, academic study, and professional clinical reference. It does not substitute for patient-specific clinical judgement, qualified specialist oncology/medical supervision, or official CDSCO/IP prescribing information. Never self-medicate with prescription, Schedule H/H1/X, NDPS, or oncology medicines.