Phenytoin

What phenytoin is used for

Phenytoin is an anticonvulsant: it calms the bursts of electrical activity in the brain that produce seizures. It is used for epilepsy, both focal seizures (ones that begin in one part of the brain) and generalised tonic-clonic seizures (the convulsive kind that affect the whole body). It also helps prevent seizures that can start during or after surgery on the brain or nervous system, and it is used to steady some irregular heart rhythms. It controls seizures rather than curing them, so it is taken long term.

How it works, and why the timing matters

Seizures come from nerve cells firing in an uncontrolled burst. Phenytoin makes the sodium channels in those cells slow to reopen, so a bigger trigger is needed for each firing and a burst has a harder time travelling across the brain. Everyday brain activity goes on as usual.

Phenytoin only protects while its level in the blood stays steady, which is why gaps between tablets cause trouble. Treatment starts with a small amount that is raised about once a week until seizures are controlled. Cutting treatment short is risky: seizures can return or come back worse, so the dose is lowered slowly. Take the tablets with water, or with food if they unsettle your stomach.

Dilantin 100mg tablets

Dilantin is the phenytoin sold here: 100mg tablets taken by mouth, with the strength printed on the pack. Phenytoin is the ingredient and Dilantin is the brand name, so a search for either word arrives at the same tablet. Phenytoin is also made as chewable tablets, long-acting capsules and a liquid; the 100mg tablet is the form sold here. Keep the pack out of direct sunlight and away from damp.

Side effects worth knowing

Drowsiness, dizziness, unsteadiness, headache, double vision, nausea and shaky hands are the usual complaints, and they often settle as the body adjusts. Gum swelling is a known phenytoin effect that can develop after months of use, particularly where plaque collects along the gum line; longer term, extra hair growth and coarser facial features can appear.

A rash is the sign to take seriously: phenytoin can cause blistering skin reactions, and a spreading rash with fever, mouth sores or peeling skin needs urgent treatment. The liver and blood can be affected too — unexplained bruising or bleeding, or yellowing of the skin and eyes, are the signs to watch for.

What changes phenytoin levels

Phenytoin is easily pushed up or down by other medicines. Antacids containing calcium, magnesium or aluminium, stomach acid reducers such as cimetidine and famotidine, aspirin-type painkillers, folic acid, vitamin D and St John's wort can all change how much stays in the blood, and alcohol shifts it too. Phenytoin works in the other direction as well, speeding up the breakdown of other medicines, so a change in your regular medicines can move the phenytoin level with it.