Phenytoin

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Phenytoin: a simple guide

What it is
Phenytoin is an anticonvulsant medicine used to prevent seizures. It’s sold under brand names like Dilantin and is also available as a generic drug. A related IV form called fosphenytoin is used in emergencies.

What it’s used for
- Prevents tonic-clonic (grand mal) seizures and focal (partial) seizures.
- Does not treat absence seizures.
- The IV form can be used for ongoing seizures that don’t respond to benzodiazepines.
- Sometimes used for certain heart rhythm problems or nerve pain, but that’s less common.

How you take it and how it works
- It can be taken by mouth or given by IV.
- IV phenytoin starts working within about 30 minutes; effects can last about a day.
- It works mainly by blocking sodium channels in the brain, which helps stop excessive seizure activity.
- It mainly acts in the motor areas of the brain to reduce spread of seizures.

Dosing and safety notes
- Blood levels are usually checked to set the right dose.
- Phenytoin has non-linear (mixed-order) elimination: small dose increases can cause big jumps in blood levels, so careful monitoring is needed.
- If given by IV, go slowly. Rapid IV administration can cause very low blood pressure and abnormal heart rhythms.
- Do not stop taking phenytoin suddenly without talking to a doctor, because seizures can come back or worsen.

Common and serious side effects
- Common: nausea, stomach upset, loss of appetite, poor coordination, increased hair growth, and gums becoming enlarged.
- Serious: drowsiness, mood changes or suicidal thoughts, liver problems, suppression of bone marrow, very low blood pressure, and severe skin reactions (rare but serious).
- Long-term use can cause gum overgrowth, numbness or tingling, and may be linked to bone density loss.
- It can cause folate deficiency leading to certain types of anemia and, in pregnancy, birth defects.
- It may affect vitamin D and calcium levels, contributing to weaker bones.
- It can interact with many other medicines (for example, warfarin and some antibiotics can raise phenytoin levels). It also induces liver enzymes, which can speed up the breakdown of other drugs.
- Alcohol can interfere with how phenytoin works.

Pregnancy and breastfeeding
- Phenytoin can increase the risk of birth defects (roughly 6% of exposed pregnancies show defects such as neural tube and heart problems), and some children may have developmental effects. Because of this, doctors try to use the smallest effective dose and may avoid combining phenytoin with other epilepsy medicines when possible.
- It is generally considered safe to breastfeed while taking phenytoin, but discuss this with a clinician.
- Folate supplementation may help reduce certain dental/gum and blood-related risks in children taking phenytoin.

Special situations and extras
- If used for a long time at high doses, there can be brain and nerve effects, including changes in coordination and balance.
- Phenytoin is a known teratogen and can be linked to birth defects; careful planning and monitoring during pregnancy are important.
- It’s on the World Health Organization’s list of essential medicines.
- Some topical/phenytoin products have been studied for wound healing, with mixed results.

A quick reminder
Phenytoin has many benefits for controlling seizures, but it requires careful dosing and monitoring because of possible side effects and interactions with other medicines. If you’re taking phenytoin or starting it, work closely with your healthcare provider to use the lowest effective dose and to watch for any problems.