phenytoin at a glance
Medicine class
Antiseizure treatment; hydantoin derivative
Main licensed uses
Focal and tonic-clonic seizures, and seizures associated with neurosurgery or severe head injury
Consistency matters
People treated for epilepsy should usually remain on the same manufacturer's preparation
Stopping treatment
Do not stop suddenly; abrupt withdrawal may trigger seizures or status epilepticus
What is phenytoin used for?
Selected products are licensed for:
Control of focal seizures, including temporal-lobe seizures.
Control of tonic-clonic seizures or a combination of focal and tonic-clonic seizures.
Prevention and treatment of seizures during or after neurosurgery or severe head injury.
Second-line treatment of trigeminal neuralgia when carbamazepine is ineffective or not tolerated.
How does phenytoin work?
Phenytoin stabilises excitable nerve-cell membranes, principally by limiting repetitive firing through voltage-gated sodium channels. This helps prevent seizure activity spreading through the brain.
phenytoin preparations and strengths
Oral suspension
Route: Oral
Strengths: 30 mg/5 mL
How to take phenytoin
Follow the product label and prescriber's instructions. Do not change manufacturer or formulation without clinical review because absorption and exposure can differ.
Shake oral suspension vigorously before measuring with an appropriate oral measuring device.
Infatabs must be chewed before swallowing.
Do not stop suddenly unless urgent substitution is directed because of a serious reaction.
phenytoin side effects
Common or expected effects
- Dizziness, drowsiness or headache
- Nausea, vomiting or constipation
- Unsteadiness, tremor or coordination problems
- Enlargement of the gums
- Increased facial or body hair
Get urgent medical advice
- Severe allergic reaction or angioedema
- Blistering, peeling or widespread severe skin reaction
- Rash with fever, swollen glands or organ inflammation, suggesting DRESS
- Unexplained bruising, bleeding, pallor, fever or severe sore throat
- Jaundice or other evidence of liver injury
- Marked confusion, slurred speech, unusual eye movements or severe unsteadiness suggesting toxicity
- New distressing or suicidal thoughts
phenytoin in pregnancy
Phenytoin crosses the placenta and is associated with an increased risk of congenital malformations and possible adverse developmental outcomes. Anyone planning pregnancy or becoming pregnant should obtain prompt specialist review, but should not stop phenytoin abruptly because uncontrolled seizures can harm both mother and baby.
phenytoin while breastfeeding
Phenytoin enters breast milk in low concentrations, but breastfeeding is not recommended in the selected product information. Discuss feeding options and antiseizure treatment with the specialist team rather than stopping treatment independently.
phenytoin interactions
Phenytoin has many clinically important interactions because other treatments can alter its concentration and it strongly induces liver enzymes.
Hormonal contraceptives
Phenytoin can reduce contraceptive effectiveness; discuss an appropriate alternative method.
Azole antifungals, macrolides and co-trimoxazole
Some can raise phenytoin concentrations and increase toxicity risk.
St John's wort
It can lower phenytoin concentrations and reduce seizure control; concomitant use should be avoided.
Common questions about phenytoin
Answers are fully visible for fast scanning and source review.
What is phenytoin used for?
It is mainly used to control focal and tonic-clonic seizures and to prevent or treat seizures associated with neurosurgery or severe head injury. Selected products are also licensed as second-line treatment for trigeminal neuralgia.
Can phenytoin be stopped suddenly?
No. Abrupt withdrawal can trigger seizures or status epilepticus; changes should be clinically supervised.
Why stay with the same manufacturer?
Sources
These sources were used to prepare and review this medicine page.
Prepared and reviewed by the iatroX editorial team.