Dilantin
Meaning of Dilantin: A long established seizure medicine whose blood level climbs unpredictably with dose, which is why it is measured rather than assumed.
Definition of Dilantin
Dilantin is a brand name for phenytoin, an anticonvulsant used to prevent and to stop seizures. It acts on sodium channels in nerve cell membranes, holding them shut for longer after each electrical impulse, which limits the rapid repeated firing a seizure depends on. It is taken by mouth for ongoing control and given into a vein when a seizure has to be stopped quickly.
Which seizures does it treat?
Phenytoin acts on seizures that spread through networks of neurons firing repeatedly, which covers focal seizures and generalized tonic-clonic seizures, including those occurring as part of epilepsy. It is also used to halt prolonged seizure activity in hospital and to lower the risk of early seizures after a serious head injury or brain surgery. It does not help absence seizures, the brief blank spells that arise by a different mechanism, and it can worsen some generalized seizure types, so the kind of seizure governs whether it is the right drug at all.
Why are blood levels checked?
Phenytoin is unusual in how the body clears it. The enzymes that break it down become saturated within the useful dose range, so once they are working at capacity a small increase in dose produces a disproportionately large jump in blood level. That behavior, combined with a narrow gap between the level that controls seizures and the level that causes trouble, is why measurement is routine rather than a sign that something has gone wrong. Most of the drug in blood is also bound to albumin and only the unbound fraction is active, so a total level can look reassuring in someone with a low albumin while the active portion is high.
What effects does it have beyond the brain?
| Effect | Description |
|---|---|
| Overgrowth of the gums | Thickened gum tissue developing with long term use |
| Coarsened facial features and extra body hair | Slow changes over prolonged treatment |
| Unsteadiness, slurred speech, flickering eye movements | Appear as the level climbs above the intended range |
| Rash | Ranges from mild to rare severe skin reactions, with risk linked to certain inherited tissue types |
| Interaction with other medicines | Phenytoin speeds the breakdown of many drugs, lowering their effect |
The interaction list is the largest practical problem with the drug. Phenytoin switches on liver enzymes that clear other medicines, so hormonal contraceptives, blood thinners, some antibiotics, transplant medicines and several other anticonvulsants lose potency alongside it, while drugs that inhibit those same enzymes push phenytoin levels up. It also affects vitamin D handling and bone density over long periods, and use in pregnancy is associated with a recognized pattern of birth defects. These properties are why newer anticonvulsants have taken over much routine prescribing, while phenytoin remains widely used where cost matters and in the intravenous setting. A side effect profile this broad is the trade for a drug that works reliably and fast.
Also known as
Used in a sentence
Dilantin was continued following brain surgery to reduce the risk of postoperative seizures.
Good to know
Phenytoin requires blood levels to remain within a narrow effective range, so periodic monitoring is a routine part of long term treatment rather than a sign that something has gone wrong.