phenytoin sodium at a glance
Medicine class
Hydantoin antiseizure medicine
Main role
Controls selected focal and tonic-clonic seizures
Consistency
People treated for epilepsy should usually remain on the same manufacturer's product
Administration
Swallow oral tablets whole with water; they may be taken before or after food
What is phenytoin sodium used for?
Selected UK products are licensed for:
Control of tonic-clonic seizures and focal seizures, including combinations of these seizure types.
Prevention or treatment of seizures during or after neurosurgery or severe head injury.
Hospital treatment of tonic-clonic status epilepticus using injection.
Second-line treatment of trigeminal neuralgia when carbamazepine is ineffective or not tolerated, for selected oral products.
Treatment of certain cardiac dysrhythmias unresponsive to usual treatment or cardioversion, for the selected Hospira injection.
How does phenytoin sodium work?
Phenytoin stabilises voltage-gated sodium channels in nerve cells. This limits sustained repetitive firing and helps stop seizure activity spreading from its point of origin.
phenytoin sodium preparations and strengths
Capsule
Route: Oral
Strengths: 25 mg, 50 mg, 100 mg, 300 mg
Tablet
Route: Oral
Strengths: 100 mg
How to take phenytoin sodium
Follow the dispensing label and use the same manufacturer's product unless a clinician has planned a change.
Swallow tablets whole with plenty of water; selected tablets may be taken before or after food.
Do not stop phenytoin suddenly without clinical advice because seizures may recur or worsen.
Tell the clinical team if phenytoin must be given through an enteral feeding tube, because feeds can substantially reduce absorption.
Check with a pharmacist before starting, stopping or changing any prescription, over-the-counter or herbal product.
phenytoin sodium side effects
Common or expected effects
- Unsteadiness or reduced coordination
- Abnormal eye movements
- Dizziness or drowsiness
- Slurred speech or confusion
- Headache, tremor or altered sensation
- Nausea, vomiting or constipation
- Gum enlargement or increased body or facial hair
Get urgent medical advice
- Severe rash, blistering, peeling, mouth sores or rash with fever and swollen glands
- Facial, mouth or throat swelling, wheezing or difficulty breathing
- Unusual bruising or bleeding, pallor, fever or severe sore throat
- Yellow skin or eyes, or other symptoms of liver injury
- Marked unsteadiness, double vision, abnormal eye movements, slurred speech or confusion suggesting toxicity
- New distressing thoughts, major mood changes or suicidal thoughts
- With injection: low blood pressure, abnormal heart rhythm, breathing difficulty or severe tissue injury around the injection site
phenytoin sodium in pregnancy
Phenytoin exposure during pregnancy is associated with an increased risk of major congenital malformations, and adverse neurodevelopmental effects cannot be excluded. Anyone planning pregnancy or who may be pregnant should contact their specialist promptly for treatment review, but should not stop phenytoin suddenly. Phenytoin can reduce the effectiveness of hormonal contraception, and folic acid use requires coordinated advice because it can lower phenytoin concentrations.
phenytoin sodium while breastfeeding
Selected oral product information does not recommend breastfeeding during phenytoin treatment because small amounts can enter breast milk. Discuss feeding options with the specialist, obstetric or infant-care team rather than stopping treatment independently.
phenytoin sodium interactions
Phenytoin has many clinically important interactions because other substances can alter its concentration and it strongly induces liver enzymes.
Hormonal contraceptives
Phenytoin may reduce contraceptive effectiveness, so alternative effective contraception should be discussed.
St John's wort
It can lower phenytoin concentrations and reduce seizure control; the combination should be avoided and stopping it may also alter phenytoin levels.
Folic acid
Folic acid can lower phenytoin concentrations, so coordinated clinical and blood-level monitoring may be needed when it is started or stopped.
Common questions about phenytoin sodium
Answers are fully visible for fast scanning and source review.
What is phenytoin sodium used for?
It is mainly used to control selected focal and tonic-clonic seizures. Particular products are also licensed for hospital seizure emergencies, seizures associated with neurosurgery or severe head injury, second-line trigeminal neuralgia and, for one selected injection, certain cardiac dysrhythmias.
Can phenytoin sodium treat absence or myoclonic seizures?
It is not effective for absence seizures and may aggravate absence or myoclonic seizures.
Sources
These sources were used to prepare and review this medicine page.
Prepared and reviewed by the iatroX editorial team.