What Is Dilantin? Uses, Dosage, and What You Need to Know in 2026

Dilantin is one of the oldest anti-seizure medications still in widespread use today. First approved by the FDA in 1939, it has been treating epilepsy for nearly nine decades. If you or a loved one has been prescribed Dilantin, here is everything you need to know — from what it is and how it works, to dosing details and important safety information.

What Is Dilantin?

Dilantin is the brand name for phenytoin sodium, an anticonvulsant (anti-seizure) medication. It belongs to the hydantoin class of drugs and is used to control certain types of seizures in people with epilepsy. The generic version is simply called phenytoin, and another brand — Phenytek — is also available.

Phenytoin is currently manufactured by Viatris (brand Dilantin) and multiple generic manufacturers. It is classified by the FDA as a narrow therapeutic index (NTI) drug, meaning the difference between an effective dose and a toxic dose is small — requiring careful monitoring.

What Is Dilantin Used For?

Dilantin is FDA-approved for the following uses:

Tonic-clonic (grand mal) seizures: Seizures that cause stiffening of the body, loss of consciousness, and rhythmic jerking movements of the limbs

Complex partial seizures (psychomotor/temporal lobe seizures): Seizures originating in one part of the brain that impair consciousness and may cause unusual behaviors or automatisms

Seizure prevention in neurosurgery: Prevention and treatment of seizures during or after brain or spinal cord surgery

Important: Dilantin is not effective for absence (petit mal) seizures. If both tonic-clonic and absence seizures are present, combined therapy is needed.

What Forms Does Dilantin Come In?

Dilantin is available in several formulations — each with important differences:

Extended-release capsules (Dilantin Kapseals): 30 mg and 100 mg strengths. These have a slow, extended absorption rate with peak blood levels 4–12 hours after dosing. They can be dosed once daily in stable adult patients on 300 mg/day.

Chewable tablets (Dilantin Infatabs): 50 mg tablets. These use the free acid form of phenytoin (not the sodium salt). They are immediate-release and must be taken 2–3 times daily.

Oral suspension (Dilantin-125): 125 mg per 5 mL. Also uses the free acid form. Useful for patients who cannot swallow capsules. Note that this formulation has faced supply issues through 2025–2026.

Injectable solution: 50 mg/mL solution used in hospital settings for status epilepticus or seizure prophylaxis around surgery. Administered by healthcare professionals only.

What Are the Standard Dilantin Doses?

Dosing is individualized based on blood level response. Typical doses include:

Adults (starting dose): One 100 mg extended-release capsule three times daily (300 mg/day total)

Adults (maintenance): 300–600 mg/day, adjusted to achieve a therapeutic serum level of 10–20 mcg/mL. If controlled at 300 mg/day, once-daily dosing may be used.

Children: 5 mg/kg/day in 2–3 divided doses initially; maintenance 4–8 mg/kg/day, not to exceed 300 mg/day. Children over 6 and adolescents may require the minimum adult dose.

It takes 7–10 days to reach steady-state blood levels when starting therapy or changing doses. Blood level monitoring is essential — do not change your dose without consulting your doctor.

How Should You Take Dilantin?

Swallow extended-release capsules whole — do not crush, chew, or open them

Take at the same time each day to maintain stable blood levels

Shake the oral suspension well before measuring — use the provided dosing syringe, not a kitchen spoon

Do not take antacids within 2–3 hours of a Dilantin dose (antacids containing calcium, magnesium, or aluminum can reduce absorption)

If you miss a dose, take it as soon as you remember — unless it's almost time for your next scheduled dose. Never double up.

Never stop taking Dilantin abruptly — this can cause status epilepticus (a dangerous prolonged seizure state)

Key Monitoring Requirements

Because of phenytoin's narrow therapeutic index, regular blood tests are required:

Serum phenytoin level: Target range 10–20 mcg/mL (total). Drawn as a trough (just before your next dose).

Complete blood count (CBC): Periodic check for blood cell changes

Liver function tests (LFTs): To monitor for hepatotoxicity

Vitamin D and bone density: With long-term use, due to phenytoin's effect on vitamin D metabolism

For a complete discussion of side effects, see our guide: Dilantin Side Effects: What to Expect and When to Call Your Doctor.

If you're having trouble filling your Dilantin prescription, medfinder can help locate which pharmacies near you have it in stock.