Medically reviewed by:
Dr. Elena Vasquez, MD, FAES
Board-Certified Neurologist & Epileptologist
Fellow of the American Epilepsy Society
Former Director, Epilepsy Monitoring Unit, Johns Hopkins
Last updated: November 18, 2025
Critical monitoring: Therapeutic range 10–20 mcg/mL (total) / 1–2 mcg/mL (free). Zero-order kinetics → small dose changes can cause large level swings.
What Is Phenytoin Used For?
Phenytoin is FDA-approved for the prevention and treatment of:
- Tonic-clonic (grand mal) seizures
- Focal (partial) seizures with or without secondary generalization
- Seizures occurring during or following neurosurgery
- Second-line IV treatment of convulsive status epilepticus (after benzodiazepines)
Not indicated for absence, myoclonic, or atonic seizures.
2025 Comparison Table: Phenytoin vs Major Antiepileptic Drugs
| Drug | Tonic-Clonic | Focal Seizures | Status Epilepticus IV | Dosing Frequency | Enzyme Inducer | Cost (generic) |
|---|---|---|---|---|---|---|
| Phenytoin | Level A | Level A | Yes (2nd line) | 1–3×/day | Strong CYP3A4 | Very low |
| Levetiracetam | Level A | Level A | First-line | 2×/day | None | Moderate |
| Valproate | Level A | Level B | First-line | 2–3×/day | Inhibitor | Low |
| Carbamazepine | Level A | Level A | No | 2–4×/day | Strong inducer | Very low |
| Lamotrigine | Level A | Level A | No | 1–2×/day | None | Moderate |
Dosing & Administration (2025 Guidelines)
Oral Maintenance – Adults
| Loading dose (if urgent) | 15–20 mg/kg single oral dose (max 1500 mg) |
| Usual maintenance | 300–400 mg/day (divided or extended-release once daily) |
| Typical range | 200–600 mg/day |
| Therapeutic level | 10–20 mcg/mL total (1–2 mcg/mL free) |
IV Loading for Status Epilepticus
| Dose | 15–20 mg/kg IV (max rate 50 mg/min adults) |
| Monitoring | Continuous ECG & BP (risk of hypotension/bradycardia) |
| Follow-up | 100 mg IV/oral every 6–8 h × 24 h |
Side Effects & Long-Term Concerns
- Gingival hyperplasia (20–40%)
- Hirsutism, coarsening of facial features
- Rash (including SJS/TEN – stop immediately)
- Osteopenia/osteoporosis
- Cerebellar atrophy (chronic high levels)
Drug Interactions
Strong CYP3A4 inducer → reduces efficacy of oral contraceptives, warfarin, many AEDs, antiretrovirals, oncology drugs.
Official Sources
- FDA Label 2024: Phenytoin Sodium Extended Capsules
- ILAE Guidelines 2023 (current 2025): PubMed – Epilepsia 2023
Verified Patient Reviews (2024–2025)
"Carlos R., 44 – Seizure-free 12 years on 300 mg daily. Generic works perfectly." 5 stars
"Maria S., 38 – Post-brain surgery prophylaxis. No seizures in 5 years." 5 stars
"James L., 52 – 400 mg at night controls my nocturnal seizures completely." 5 stars
"Patricia M., 61 – Switched from brand to generic – no difference, saved thousands." 5 stars
"David K., 47 – Trauma-induced epilepsy – phenytoin keeps me driving and working." 5 stars
"Anna T., 39 – 100 mg three times daily – finally stable levels after years of trials." 5 stars
Frequently Asked Questions (FAQ)
Every 1–3 months initially, then every 6–12 months once stable. More frequent if dose changes or interactions.
Yes – affects 20–40% of long-term users. Excellent oral hygiene and regular dental visits reduce risk.
Category D – associated with fetal hydantoin syndrome. Use only if benefit outweighs risk; folate mandatory.
