Dilantin is a brand-name seizure medication containing the drug phenytoin. It is approved to treat two specific seizure types: tonic-clonic seizures (formerly called grand mal seizures) and psychomotor seizures, which originate in the temporal lobe of the brain. It is also used to prevent and treat seizures that occur during or after brain surgery.
Though Dilantin has been a cornerstone of epilepsy treatment for decades, its role has shifted as newer medications have emerged. Understanding what it does, how it works, and what to watch for can help you make sense of why it might be prescribed and what to expect.
How Dilantin Controls Seizures
Seizures happen when neurons in the brain fire electrical signals too rapidly and in uncontrolled bursts. Dilantin works by stabilizing the channels that allow sodium to flow into nerve cells. When these sodium channels are overactive, neurons fire repeatedly. Phenytoin encourages these channels to stay in a resting, inactive state for longer, which slows the rapid-fire signaling that triggers a seizure. It doesn’t sedate the brain broadly the way some older medications do. Instead, it selectively dampens the abnormal, high-frequency electrical activity while leaving normal signaling relatively intact.
Off-Label Uses Beyond Epilepsy
Phenytoin was historically one of the first drugs used for trigeminal neuralgia, a condition that causes intense, stabbing facial pain along the trigeminal nerve. It is rarely used as a long-term preventive for this condition today because of side effects associated with prolonged use, but it still has a niche role in acute flare-ups. In hospital settings, an intravenous form (fosphenytoin) can be given to rapidly bring severe trigeminal neuralgia pain under control. Some clinicians also prescribe it for certain types of nerve pain, though newer alternatives are generally preferred.
Why Dilantin Is Less Common Than It Used to Be
For many years, phenytoin was the most frequently prescribed seizure medication in the world. That has changed considerably. Several newer drugs, including lamotrigine, levetiracetam, oxcarbazepine, and lacosamide, have matched phenytoin’s effectiveness in head-to-head trials while causing fewer side effects and being easier to dose. Levetiracetam in particular is now widely used as a first-line treatment for both focal seizures and generalized tonic-clonic seizures.
A major reason for the shift is that Dilantin has what’s called a narrow therapeutic window. The target blood level sits between 10 and 20 mcg/mL. Below 10, seizures may not be adequately controlled. Above 30, severe toxicity can occur. That leaves relatively little room for error. Complicating things further, phenytoin follows nonlinear metabolism, meaning a small increase in dose can cause a disproportionately large jump in blood levels. This makes it tricky to keep levels in the safe, effective range without frequent blood draws.
Side Effects to Know About
Dilantin has a few side effects that are fairly unique among seizure medications. The most distinctive is gum overgrowth, technically called gingival hyperplasia. Up to 50% of people taking phenytoin experience some degree of this. The gum tissue gradually enlarges and can grow over the teeth, leading to bleeding, discomfort while chewing or brushing, and cosmetic concerns. The severity tends to be dose-dependent, so higher blood levels increase the risk. Good oral hygiene helps but doesn’t always prevent it.
Excess hair growth on the arms and legs (hypertrichosis) is another recognizable side effect. Other common issues include dizziness, drowsiness, difficulty with coordination, and slurred speech, especially when blood levels creep toward the upper end of the therapeutic range. Long-term use can also cause peripheral neuropathy, a type of nerve damage that produces tingling, numbness, or weakness in the hands and feet.
Blood Monitoring and Why It Matters
If you take Dilantin, expect regular blood tests. Your care team will check phenytoin levels to make sure they stay within that 10 to 20 mcg/mL target. They may also monitor liver function, kidney markers, blood counts, and vitamin D levels over time. Phenytoin can reduce vitamin D absorption, which may affect bone health with long-term use.
One detail that matters more than most people realize: albumin levels. Phenytoin binds heavily to a protein in your blood called albumin. If your albumin is low, which can happen in older adults, people with liver disease, or during pregnancy, more of the drug floats freely in the bloodstream. This means the standard blood level reading can look normal while the actual active drug concentration is higher than expected. In these situations, your doctor may order a “free phenytoin” level instead. The therapeutic range for free phenytoin is much narrower: 1.0 to 2.0 mcg/mL, with levels above 2.5 considered critical.
Drug Interactions
Dilantin is one of the most interaction-prone medications in common use. It is a strong inducer of a major liver enzyme system called CYP3A and a moderate inducer of two others (CYP1A2 and CYP2C19). In practical terms, this means phenytoin speeds up the rate at which your liver breaks down many other drugs. Medications metabolized through these pathways can lose a significant portion of their effectiveness. Birth control pills, blood thinners, certain heart medications, immunosuppressants, and many other drugs are affected.
The reverse is also true. Other medications can raise or lower phenytoin levels unpredictably. Because of the narrow therapeutic window, even modest changes in blood concentration can push you into either under-treatment or toxicity. If you’re prescribed a new medication while taking Dilantin, your phenytoin levels will likely need to be rechecked.
Dilantin and Pregnancy
Phenytoin carries well-documented risks during pregnancy. A condition called fetal hydantoin syndrome affects roughly 5% to 11% of babies exposed to phenytoin in the womb. Features can include a cleft lip or palate, widely spaced eyes, a flattened nasal bridge, a smaller-than-expected head size, and underdeveloped fingers, toes, or nails. Developmental delays and intellectual disabilities may also appear later in childhood.
That said, stopping seizure medication abruptly during pregnancy is dangerous for both the parent and the developing baby. Uncontrolled seizures pose their own serious risks. If you’re planning a pregnancy and currently take Dilantin, your doctor will typically work with you beforehand to adjust your medications, potentially switching to an alternative with a lower risk of birth defects. Folic acid supplementation before and during pregnancy is also standard practice to help reduce the chance of congenital malformations.
Extended-Release vs. Prompt-Release Forms
Dilantin comes in several forms, and they are not interchangeable without careful monitoring. The extended-release capsule is absorbed slowly, reaching peak blood levels in 4 to 12 hours. This is the only form designed for once-daily dosing. Prompt-release capsules absorb much faster, peaking in about 1.5 to 3 hours, and typically need to be taken multiple times a day.
There’s also a liquid suspension and chewable tablets, both of which use a slightly different chemical form of phenytoin (the free acid rather than the sodium salt). Because the free acid form contains about 8% more active drug by weight, switching between formulations without adjusting the dose can push blood levels higher or lower than expected. Any switch between Dilantin products should come with blood level monitoring to make sure you stay in the therapeutic range.