Information last reviewed: May 2026 — for educational purposes only.
What Is Dilantin (Phenytoin)?
Phenytoin, marketed as Dilantin, is a first-generation hydantoin antiepileptic used since the 1930s. Its primary mechanism is blockade of voltage-gated sodium channels in their inactive state, preventing repetitive neuronal firing. It has a narrow therapeutic index and demonstrates non-linear (zero-order) pharmacokinetics at higher doses — small dose increases can cause disproportionately large rises in plasma concentration, greatly increasing toxicity risk. Therapeutic drug monitoring (serum phenytoin levels) is mandatory. Free (unbound) phenytoin levels are used in hypoalbuminaemia or renal failure. Available as extended-release capsules for once- or twice-daily dosing, standard-release capsules, oral suspension, chewable tablets, and IV solution.
Prescription (Rx) Status
Phenytoin is prescription-only. It has numerous clinically significant drug interactions (induces CYP1A2, 2C9, 2C19, 3A4), affecting many co-administered medications. Fosphenytoin (Cerebyx) is the IV/IM prodrug preferred over IV phenytoin for status epilepticus due to safer cardiovascular profile. FDA teratogen category D — associated with fetal hydantoin syndrome.
Uses of Phenytoin
FDA-approved for generalised tonic-clonic (grand mal) seizures, complex partial seizures, and prevention of seizures during neurosurgery. Off-label: trigeminal neuralgia and certain cardiac arrhythmias (IV phenytoin — largely replaced by other agents).
Capsule Strengths
Extended-release capsules (Dilantin Kapseal): 30 mg and 100 mg. Oral suspension: 125 mg/5 mL. Chewable tablets (Infatabs): 50 mg. Target serum levels: 10–20 mcg/mL (total phenytoin). Usual adult maintenance dose: 300–400 mg/day. Slow titration required. Different formulations are NOT interchangeable without reassessment, as brands differ in bioavailability.
Price
Generic phenytoin is very affordable (~$10–$30/month). Switching generics or brands requires level monitoring.
Frequently Asked Questions
Q: What are the signs of phenytoin toxicity?
A: Characteristic triad of nystagmus, ataxia (unsteady gait), and dysarthria (slurred speech). At higher levels: confusion, lethargy, cardiovascular effects, and coma. Levels above 20 mcg/mL are generally toxic.
Q: Does Dilantin cause gum problems?
A: Yes — gingival hyperplasia (gum overgrowth) is a hallmark long-term side effect, occurring in ~20% of patients. Good dental hygiene reduces severity. It improves after discontinuation.
Q: Are different phenytoin formulations interchangeable?
A: No. Extended-release and immediate-release formulations differ in absorption rate. Switching formulations requires reassessment of serum levels, dosing schedules, and toxicity watch. Always consult the prescriber before switching.
Disclaimer: This page is for informational purposes only and does not constitute medical advice. Phenytoin requires a valid prescription. Always consult a qualified healthcare professional. See our full disclaimer.