Phenytoin is one of the oldest anti-seizure medications still in regular use, first introduced in the late 1930s after researchers discovered it could stop convulsions without the heavy sedation caused by barbiturates, which were the standard treatment at the time. It works by calming overactive electrical signaling in the brain, and for decades it was the go-to drug for most forms of epilepsy. Phenytoin remains effective and widely prescribed, but it is also one of the trickiest medications to dose correctly, with a reputation among clinicians for unpredictable blood levels, a long list of drug interactions, and side effects that range from cosmetic annoyances to serious organ damage.
How Phenytoin Was Discovered
Before phenytoin, people with epilepsy were largely treated with phenobarbital and other barbiturates, drugs that controlled seizures but left patients drowsy and sedated. Phenytoin had actually been synthesized decades earlier, but when early screening showed it did not produce sedation the way barbiturates did, the pharmaceutical company Parke-Davis set it aside as apparently useless. The breakthrough came in 1936, when two researchers named Putnam and Merritt developed a new animal model for testing anticonvulsant drugs and found that phenytoin could prevent electrically induced seizures in cats without knocking them out. They went on to confirm the drug’s value in patients between 1937 and 1940, fundamentally changing the treatment of epilepsy.1PubMed. Phenytoin: 80 years young, from epilepsy to breast cancer, a remarkable molecule with multiple modes of action
How It Controls Seizures
Seizures happen when groups of neurons in the brain start firing in rapid, synchronized bursts. Phenytoin targets voltage-gated sodium channels, which are the tiny pores on nerve cells that allow sodium to rush in and trigger an electrical impulse. Specifically, phenytoin latches onto these channels when they are in an inactivated state and holds them there longer than normal, making it harder for neurons to fire again in quick succession.2PubMed Central. Voltage-gated sodium channels: pharmaceutical targets via anticonvulsants to treat epileptic syndromes The result is that the runaway chain reaction of a seizure gets interrupted without completely shutting down normal brain activity. This same mechanism also explains why phenytoin occasionally shows up in the treatment of certain heart rhythm problems, since the heart also relies on sodium channels to coordinate its electrical signals.
Why Dosing Is So Difficult
Phenytoin is classified as a narrow therapeutic index drug, meaning the gap between a dose that works and a dose that causes toxicity is uncomfortably small. The generally accepted therapeutic blood level sits between 10 and 20 micrograms per milliliter, and the ratio between the minimum effective concentration and the minimum toxic concentration is only about two-fold, comparable to high-risk drugs like warfarin.3American Epilepsy Society. Classification of Phenytoin as a Narrow Therapeutic Index Drug That leaves very little room for error.
Making matters worse, phenytoin does not follow the usual rules of drug metabolism. Most medications are cleared from the body at a rate proportional to how much is present: double the dose, and the body clears it roughly twice as fast. Phenytoin instead saturates the liver enzymes responsible for breaking it down, even at concentrations within the therapeutic range. Once those enzymes are working at full capacity, any extra drug just accumulates.4Australian Prescriber. Pharmacokinetics made easy 9: Non-linear pharmacokinetics This means a small increase in dose can produce a disproportionately large jump in blood levels, tipping a patient from well-controlled seizures into toxicity. Dose adjustments are typically made in small increments of 30 to 100 milligrams per day, and clinicians rely heavily on blood tests to track where levels actually land.5American Epilepsy Society. Classification of Phenytoin as a Narrow Therapeutic Index Drug
Protein Binding and Free Drug Levels
About 90 percent of phenytoin circulating in the blood is bound to a protein called albumin. Only the unbound (“free”) fraction actually reaches the brain and does the work. This creates a practical problem: standard blood tests measure total phenytoin, bound and unbound combined. If your albumin level is low, more of the drug floats free, meaning the active concentration could be much higher than the total number suggests.
Patients with low albumin, a common finding in older adults, people with liver disease, and critically ill hospitalized patients, are especially vulnerable. One study found that when albumin fell below 3.5 grams per deciliter, the calculated and measured free phenytoin levels diverged by 20 percent or more far more often than they did in patients with normal albumin.6PubMed Central. Differences between the measured and calculated free serum phenytoin concentrations in epileptic patients The binding itself is also concentration-dependent, adding another layer of unpredictability.7PubMed Central. Nonlinear protein binding of phenytoin in clinical practice: Development and validation of a mechanistic prediction model For patients with both low albumin and kidney problems, measuring the free phenytoin level directly is generally recommended rather than relying on equations that estimate it from the total level.8PubMed. Predicting Unbound Phenytoin Concentrations: Effects of Albumin Concentration and Kidney Dysfunction
Side Effects at Therapeutic and Toxic Levels
Even when blood levels are technically in range, phenytoin can cause a number of side effects with long-term use. One of the most visible is gingival overgrowth, a condition where the gum tissue swells and grows over the teeth. This happens because phenytoin alters the way gum tissue handles collagen and other structural components, leading to an accumulation of connective tissue.9PubMed Central. Phenytoin-induced gingival overgrowth: a review of the molecular, immune, and inflammatory features The effect ranges from mild thickening to severe overgrowth that interferes with eating and oral hygiene. Good dental care helps, but some patients require surgical trimming of the tissue.
When levels climb above the therapeutic range, the central nervous system bears the brunt. Nystagmus, a rhythmic flickering of the eyes, is often the first warning sign. As levels continue to rise, patients may develop unsteady walking, slurred speech, confusion, nausea, and in severe cases coma or even paradoxical seizures.10PubMed. Phenytoin poisoning These neurological effects are typically reversible once levels come back down, but the cardiac risks of very high levels, particularly with rapid intravenous loading, are less forgiving.
Effects on Bone Health
One of phenytoin’s less well-known consequences is its impact on bones. Long-term use lowers vitamin D levels, which in turn disrupts calcium absorption and can lead to softening of the bones. Research comparing patients on long-term anticonvulsant therapy with healthy controls found significantly lower bone mineral density in the spine region among patients, and the decrease correlated with how long they had been on therapy. Lab work showed lower levels of both major forms of vitamin D in the patient group, along with evidence of increased bone breakdown.11PubMed. Long-term anticonvulsant therapy leads to low bone mineral density–evidence for direct drug effects of phenytoin and carbamazepine on human osteoblast-like cells Beyond the vitamin D pathway, there is evidence that phenytoin may directly inhibit the growth of bone-forming cells at therapeutic concentrations, compounding the problem.
A case report illustrates how these effects can spiral. A 69-year-old woman on phenytoin for over a decade developed persistent low calcium levels that could not be corrected even with supplements, along with vitamin D deficiency and elevated parathyroid hormone. Her seizures became increasingly difficult to control. It was only after phenytoin was stopped that her calcium normalized and her seizures came under control again.12PubMed Central. Loss of Seizure Control in a Patient With Vitamin D Deficiency and Phenytoin-Induced Hypocalcemia For anyone on phenytoin long-term, periodic monitoring of vitamin D and bone density is worth discussing with a clinician.
Genetics and Severe Skin Reactions
Phenytoin can trigger severe, sometimes life-threatening, skin reactions, the most feared being Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), conditions in which the skin blisters and peels off in sheets. These reactions are rare in the general population, but certain genetic markers dramatically raise the odds. A meta-analysis of nine case-control studies found that carrying the HLA-B*15:02 gene variant increased the risk of phenytoin-induced SJS/TEN roughly five- to six-fold.13PubMed. HLA-B*1502 increases the risk of phenytoin or lamotrigine induced Stevens-Johnson Syndrome/toxic epidermal necrolysis: evidence from a meta-analysis of nine case-control studies This variant is most common in people of Southeast Asian descent, and the FDA recommends genetic testing for it before starting phenytoin in patients with that ancestry.
Beyond the HLA system, the liver enzymes that break down phenytoin also carry genetic variation that matters. People with certain CYP2C9 gene variants metabolize the drug more slowly, leading to higher blood levels at a given dose. One study found that patients with reduced CYP2C9 activity were roughly four and a half times more likely to develop skin reactions than those with normal enzyme function, even after accounting for dose, age, and ethnicity.14PubMed Central. Associations of CYP2C9 and CYP2C19 Pharmacogenetic Variation with Phenytoin-Induced Cutaneous Adverse Drug Reactions Research in Japanese patients confirmed that CYP2C9 mutations had an even greater impact on phenytoin metabolism than CYP2C19 mutations, with the most affected patients showing roughly a 40 percent reduction in the maximum rate at which their liver could clear the drug.15PubMed. The effects of genetic polymorphisms of CYP2C9 and CYP2C19 on phenytoin metabolism in Japanese adult patients with epilepsy
Drug Interactions
Phenytoin is a powerful inducer of liver enzymes, meaning it revs up the machinery that breaks down many other medications. If you are on phenytoin and your doctor adds another drug, or vice versa, there is a good chance one will affect the blood levels of the other. Phenytoin boosts the activity of several enzyme families, which can cause other drugs to be cleared faster and become less effective. Oral contraceptives, blood thinners, certain antibiotics, and many psychiatric medications can all be affected.
The interaction between phenytoin and valproic acid, another common anti-seizure drug, is a classic example of how tangled things can get. Valproic acid pushes phenytoin off its protein binding sites, freeing more of it into the bloodstream, while also inhibiting the enzymes that break phenytoin down. These two effects pull in opposite directions: the displacement initially lowers total phenytoin levels (because the freed drug gets cleared faster), but the metabolic inhibition then causes free phenytoin to accumulate over time.16PubMed. Interaction between phenytoin and valproic acid: plasma protein binding and metabolic effects Studies in epilepsy patients confirmed this biphasic pattern, with total phenytoin levels dipping initially and then free phenytoin levels climbing as the metabolic block took hold.17PubMed. Interactions of valproic acid with phenytoin Monitoring both total and free levels becomes essential when these two drugs are combined.
Intravenous Phenytoin and Purple Glove Syndrome
Phenytoin is poorly soluble in water, so the injectable formulation requires a highly alkaline solution with propylene glycol and ethanol to keep it dissolved. That harsh vehicle is the source of most problems with IV administration. The solution must be given slowly and cannot be heavily diluted, and even with careful technique it can irritate veins and surrounding tissue.18Advanced Drug Delivery Reviews. A case for prodrugs: Fosphenytoin
An uncommon but dramatic complication of IV phenytoin is purple glove syndrome, in which the hand and forearm around the infusion site develop pain, dark discoloration, and swelling, sometimes progressing to tissue death. The exact cause is not fully understood, but likely involves some combination of the drug leaking out of the vein and small clots forming in nearby blood vessels.19PubMed Central. Purple Glove Syndrome: Recognizing a Rare Complication of Intravenous Phenytoin Reported cases range from mild skin irritation to limb-threatening damage requiring surgery.20Journal of Medical Toxicology. Purple Glove Syndrome after Phenytoin or Fosphenytoin Administration: Review of Reported Cases and Recommendations for Prevention
Fosphenytoin was developed specifically to solve these problems. It is a water-soluble prodrug that the body converts into phenytoin after injection. Because it dissolves easily, fosphenytoin does not need the caustic solvents, can be given faster, and can be injected into a muscle if IV access is unavailable.21PubMed. Safety of fosphenytoin sodium In emergency settings, fosphenytoin has largely replaced IV phenytoin where it is available, though it costs significantly more.
Pregnancy and Fetal Hydantoin Syndrome
Phenytoin taken during pregnancy can cause a recognizable pattern of birth defects known as fetal hydantoin syndrome. The features were first described in 1975 and include distinctive facial characteristics (a flat nasal bridge, a short neck, low-set ears, and sometimes cleft lip or palate), underdeveloped fingertips and toenails, and smaller-than-expected head size.22PubMed Central. Fetal Hydantoin Syndrome: A Case Report Growth restriction, intellectual disability, heart defects, and hernias have also been reported. Not every exposed baby is affected. Research on a toxicity marker in amniotic fluid cells found that fetuses with low ability to detoxify a reactive phenytoin byproduct were the ones who developed the syndrome, while those with higher detoxification capacity were unaffected.23PubMed. Prenatal prediction of risk of the fetal hydantoin syndrome Even siblings exposed to the same drug levels in the womb can show different degrees of severity, as documented in a set of affected triplets.24PubMed. Fetal hydantoin syndrome in triplets. A unique experiment of nature Because of these risks, clinicians generally try to switch pregnant patients or those planning pregnancy to safer alternatives whenever possible.
Cognitive Effects
Beyond the obvious toxicity symptoms at high levels, phenytoin can subtly impair thinking even when blood concentrations are within range. A study comparing phenytoin with carbamazepine, another older anti-seizure medication, found that phenytoin had a greater negative impact on cognitive function during ongoing treatment. The reassuring finding was that these effects were reversible: after both drugs were completely withdrawn, cognitive performance returned to baseline.25JAMA Neurology. Carbamazepine and Phenytoin: Comparison of Cognitive Effects in Epileptic Patients During Monotherapy and Withdrawal For patients who notice mental fogginess, slowed processing, or memory difficulties while on phenytoin, the drug itself may be contributing, and switching to a newer agent is a reasonable conversation to have.
Phenytoin in Older Adults
Aging introduces several factors that make phenytoin management harder. Albumin levels tend to decline with age, raising the free fraction. Kidney and liver function often decrease, slowing drug clearance. And older adults are typically on more medications, multiplying the risk of interactions. Research has found that dosage requirements for phenytoin tend to decrease in elderly patients, likely due to both pharmacokinetic changes and increased sensitivity to the drug’s therapeutic and toxic effects.26PubMed. Phenytoin half-life and clearance during maintenance therapy in adults and elderly patients with epilepsy Given the cognitive side effects described above and the higher fracture risk from phenytoin’s effects on bone, many epilepsy specialists now prefer newer anti-seizure medications for older patients starting treatment for the first time.
Where Phenytoin Stands Today
Newer anti-seizure medications like levetiracetam, lamotrigine, and lacosamide have gradually displaced phenytoin as a first-line choice in many settings, largely because they are easier to dose, have fewer interactions, and carry milder side-effect profiles. In one large trial comparing levetiracetam with phenytoin as a second-line treatment for children in status epilepticus (prolonged seizures that do not stop on their own), seizures were terminated in about 70 percent of children given levetiracetam versus about 64 percent given phenytoin, though the difference was not statistically significant.27The Lancet. Levetiracetam versus phenytoin for second-line treatment of paediatric convulsive status epilepticus (EcLiPSE): a multicentre, open-label, randomised trial Phenytoin remains useful in specific situations, particularly in emergency departments and intensive care units where its long track record and well-understood pharmacology are advantages, and occasionally as an antiarrhythmic in patients who cannot tolerate other options.
Phenytoin in Veterinary Medicine
Dogs develop epilepsy too, and phenytoin was one of the first drugs tried in canine patients. It turned out to be a poor fit. After IV injection in dogs, the drug has a half-life of only about four and a half hours, far shorter than in humans. Oral absorption is erratic, with one study finding average bioavailability of only about 36 percent from tablet form. Worse, phenytoin powerfully induces its own metabolism in dogs, so blood levels that start in the therapeutic range during the first few days of treatment drop to ineffective concentrations within a week.28PubMed. Clinical pharmacokinetics of phenytoin in the dog: a reevaluation Phenobarbital and newer agents like zonisamide have largely taken over as the standard anti-seizure drugs in veterinary practice, making phenytoin one of the few medications that works well in humans but poorly in a closely related species.