Phenytoin is a long‑standing seizure medicine used for tonic‑clonic and focal seizures. It works by slowing abnormal brain signals through sodium channel blockade. That sounds technical, but what matters is this: phenytoin works for many people, yet it needs careful dosing and monitoring because small changes can have big effects.
Doctors prescribe phenytoin for generalized tonic‑clonic seizures, some focal seizures, and for short‑term control after brain surgery or severe head injury. It's not the first choice for every seizure type, but it remains useful when other drugs aren’t right or are too costly.
Key facts: phenytoin is a liver enzyme inducer — it speeds up the breakdown of other drugs. That can make birth control, warfarin, and some antidepressants less effective. If you take multiple medicines, tell your doctor and pharmacist so they can check interactions.
Phenytoin dosing is tricky because its metabolism can become saturated. That means doubling a dose can more than double blood levels. Doctors use blood tests to check total and sometimes free phenytoin levels. Typical therapeutic total levels sit around 10–20 mcg/mL, but the free (active) level matters more if you have low albumin or other conditions.
Common side effects include dizziness, unsteady walking, nystagmus (eye jerking), gum overgrowth, and excess hair growth. Serious issues — though less common — include severe rashes (like Stevens‑Johnson syndrome), liver problems, or low blood counts. If you get a spreading rash, fever, sore throat, or yellowing skin, stop the drug and call your doctor right away.
IV phenytoin must be given slowly. Rapid infusion can cause low blood pressure or heart rhythm issues. In many hospitals, fosphenytoin (a related IV form) is preferred because it causes fewer local and cardiac problems.
Pregnancy: phenytoin raises the risk of birth defects (fetal hydantoin syndrome). Women of childbearing age should discuss contraception and pregnancy planning with their clinician before starting phenytoin.
Special groups: older adults and people with low albumin or liver disease need lower doses and more monitoring. Children often need higher per‑kilogram doses because they metabolize the drug faster.
Practical tips: take phenytoin at the same time each day, consider taking with food to reduce stomach upset, don’t stop abruptly (seizures can return), and carry a list of your meds. Ask for blood level checks after dose changes or if you add new medicines. If you buy meds online, use only reputable pharmacies that require a prescription.
Questions for your provider: Do I need total or free phenytoin levels? How often should levels and blood tests be done? Is there a safer alternative for my seizure type? Keep the conversation open — small adjustments make a big difference with phenytoin.
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