R R

What Is Phenytoin?

Phenytoin is a prescription antiseizure medication used to manage certain forms of epilepsy and prevent or treat seizures associated with neurosurgery. It belongs to the hydantoin anticonvulsant class and is available in oral and injectable formulations. Extended-release phenytoin sodium capsules are commonly associated with the brand name Dilantin. Phenytoin helps reduce abnormal electrical activity in the brain but does not cure epilepsy. Because small dose changes can produce substantial differences in blood concentrations, treatment requires individualized dosing and monitoring.

Link to This Resource Page

Provide a valuable resource to your clients or customers by linking to this resource page. Just place the following link on your website.

To display this...

What Is Phenytoin?

Phenytoin is a prescription antiseizure medication used to manage certain forms of epilepsy and prevent or treat seizures associated with neurosurgery. It belongs to the hydantoin anticonvulsant class and is available in oral and injectable formulations. Extended-release phenytoin sodium capsules are commonly associated with the brand name Dilantin. Phenytoin helps reduce abnormal electrical activity in the brain but does not cure epilepsy. Because small dose changes can produce substantial differences in blood concentrations, treatment requires individualized dosing and monitoring.

read more about phenytoin ...

Copy this HTML:

Copy HTML Copied!

How Does Phenytoin Work?

Phenytoin stabilizes neuronal membranes by affecting voltage-dependent sodium channels. This limits sustained, repetitive electrical firing and helps prevent abnormal seizure activity from spreading through the brain. Its therapeutic effects depend on maintaining appropriate drug concentrations. Phenytoin has nonlinear metabolism, meaning that a small increase in dose can sometimes cause a disproportionately large increase in blood levels. The medication can also change the metabolism of other drugs through effects on liver enzymes. Its mechanism does not make it appropriate for every type of seizure disorder.

What Types of Seizures Does Phenytoin Treat?

Phenytoin is approved for tonic-clonic seizures and certain focal seizures historically described as psychomotor or temporal lobe seizures. It is also used to prevent and treat seizures occurring during or after neurosurgery. Injectable formulations have additional administration considerations for acute seizure management. Phenytoin is not generally appropriate for absence seizures and can worsen some seizure patterns. A neurologist evaluates seizure type, patient age, liver function, other medications, and pregnancy considerations before prescribing treatment. Blood concentration monitoring may help guide treatment.

How Should Phenytoin Be Taken?

Phenytoin dosing varies by formulation, age, treatment response, and blood concentration. Adults starting extended-release phenytoin sodium capsules are commonly prescribed 100 mg three times daily, with adjustments according to seizure control and tolerability. Certain stabilized patients may qualify for once-daily dosing with an appropriate extended-release formulation. Pediatric doses are based on body weight. Different phenytoin formulations are not automatically interchangeable at the same dose. Patients should take doses consistently, attend recommended blood tests, and never stop treatment abruptly because serious or prolonged seizures can occur.

What Are the Side Effects and Warnings of Phenytoin?

Common adverse effects include dizziness, sleepiness, unsteady movement, double vision, and involuntary eye movements. Long-term treatment can cause gum overgrowth, changes in hair growth, and bone-related complications. Serious risks include severe skin reactions, drug reaction with eosinophilia and systemic symptoms, liver injury, blood disorders, and suicidal thoughts or behavior. Certain genetic factors can increase the risk of serious skin reactions. Phenytoin can reduce the effectiveness of hormonal contraceptives and can harm a developing fetus. Severe rash, fever, swollen glands, or worsening coordination requires prompt assessment.

Common Questions About Phenytoin

Is phenytoin the same as Dilantin?

Dilantin is a brand name for certain phenytoin sodium formulations. Other phenytoin products can have different release characteristics and dosing instructions.

Can phenytoin cause gum overgrowth?

Yes. Gingival hyperplasia is a recognized effect of phenytoin. Regular dental care and good oral hygiene can help manage this complication.

Why are blood tests needed during phenytoin treatment?

Phenytoin has a narrow therapeutic range and nonlinear metabolism. Blood testing can help clinicians assess exposure, effectiveness, and toxicity risk.

Can phenytoin affect eyesight?

Yes. Phenytoin can cause double vision, blurred vision, or involuntary eye movements, particularly at elevated blood concentrations. Significant visual changes require medical evaluation.

References

DailyMed - PHENYTOIN SODIUM capsule, extended release. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f4353734-29b0-4027-824d-2fc97f778c67. Date Accessed October 30, 2026.

Phenytoin. LiverTox. https://www.ncbi.nlm.nih.gov/books/NBK548889/. Date Accessed October 30, 2026.

Phenytoin. MedlinePlus. https://medlineplus.gov/druginfo/meds/a682022.html. Date Accessed October 30, 2026.

Phenytoin. PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/Phenytoin. Date Accessed October 30, 2026.

Phenytoin. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK551520/. Date Accessed October 30, 2026.