Phenytoin: how it works, what it's used for, and key safety notes

Phenytoin is an older but still widely used anti-seizure medicine. It calms nerves in the brain by blocking sodium channels, so it helps stop tonic-clonic and focal seizures and is a second-line option in some emergencies. Because phenytoin behaves differently from many drugs, knowing the basics makes it easier to use safely.

Quick facts you should know

Phenytoin comes as tablets, extended-release capsules, and IV. Typical IV loading doses are around 15–20 mg/kg (given slowly); oral maintenance often falls near 300–400 mg per day split into doses, but providers tailor this a lot. Blood levels guide dosing: total plasma target is generally 10–20 µg/mL (free 1–2.5 µg/mL). Watch out: phenytoin shows nonlinear kinetics — small dose changes can cause big level swings.

Key side effects and safety

Common short-term effects include dizziness, nystagmus (eye jerking), and ataxia (clumsy movements). With longer use you may see gingival hyperplasia (gum overgrowth), excess facial hair, and bone thinning. Serious but rare problems include severe skin reactions and blood disorders, so any rash or unexplained bruising needs urgent attention. Phenytoin can cause birth defects (fetal hydantoin syndrome), so pregnancy planning and folate are topics to discuss with your doctor.

IV phenytoin has specific risks: give it slowly (usually not faster than 50 mg/min in adults) to avoid low blood pressure or heart rhythm problems. Fosphenytoin is a safer IV alternative for faster loading because it’s gentler on veins and allows quicker infusion rates expressed as phenytoin equivalents (PE).

Interactions, monitoring, and practical tips

Phenytoin interacts with many drugs. It's a strong enzyme inducer, so it can lower levels of oral contraceptives, warfarin, many statins, and some antivirals. Other anticonvulsants and antibiotics can alter phenytoin levels too. Tell every provider and pharmacist you're on phenytoin.

Monitoring matters: check serum phenytoin levels after dose changes or if you suspect toxicity, and repeat at steady state (usually 5–7 days). Periodic blood counts and liver tests are common. For long-term users, monitor bone health and dental care — good oral hygiene and regular dental visits reduce gum problems.

Practical hints: don’t stop phenytoin suddenly — that can trigger seizures. If you take it by mouth with tube feeds, separate the two (hold feeds 1–2 hours if possible) because feeding formulas can reduce absorption. Always follow your prescriber's schedule and ask about switching to newer drugs if side effects or interactions become a problem.

If you have questions about dosing, pregnancy, or combining phenytoin with other meds, talk with your prescriber or a pharmacist. They can run level checks and suggest safer alternatives when needed.