Dilantin (phenytoin) causes a wide range of side effects, from common issues like dizziness and gum overgrowth to rare but serious reactions involving the skin, liver, and heart. About half of all people taking it will experience at least one noticeable side effect, and the drug requires regular blood monitoring because the line between a therapeutic dose and a toxic one is narrow.
Common Side Effects
The side effects most people notice first are neurological. Decreased coordination, unsteadiness, slurred speech, and mental confusion are all frequently reported. Trembling, nervousness, and difficulty with fine motor control are also common. These effects tend to worsen as blood levels of the drug rise, which is why even small dose changes can make a noticeable difference in how you feel day to day.
Other common complaints include nausea, constipation, headaches, drowsiness, and trouble sleeping. Some people develop coarsening of facial features or excess body hair growth over time. These cosmetic changes are more common in people who take the drug for years.
Gum Overgrowth
Roughly half of the approximately 2 million people taking phenytoin develop some degree of gum overgrowth, making it one of the drug’s most distinctive side effects. The gums swell and grow over the teeth, sometimes severely enough to interfere with eating and oral hygiene. The overgrowth tends to start within the first few months of treatment and worsens over time.
Good oral hygiene is the most effective way to minimize it. Careful brushing, flossing, and regular dental cleanings can slow the process. If the overgrowth becomes moderate or severe, a dentist or periodontist may need to surgically trim the excess tissue. Ideally, switching to a different seizure medication resolves the problem, but that isn’t always an option.
How Toxicity Develops
Phenytoin has a narrow therapeutic window. The target blood level for most adults is 10 to 20 micrograms per milliliter. Side effects escalate in a predictable pattern as levels climb above that range:
- Above 20 mcg/mL: Involuntary eye movements (nystagmus) become common, often the earliest visible sign that levels are too high.
- Above 30 mcg/mL: Slurred speech, poor coordination, tremor, and seizures can develop. This is the threshold formally considered toxic.
- Above 40 mcg/mL: Severe lethargy, confusion, and stupor typically set in.
- Above 50 mcg/mL: Coma and seizures become likely.
Because the drug’s metabolism is unusual (small dose increases can cause disproportionately large jumps in blood levels), regular blood tests are essential. Falls and injuries from drug-induced unsteadiness are the most common real-world complication of toxicity that goes unrecognized.
Brain Changes With Long-Term Use
One of the more concerning long-term risks is shrinkage of the cerebellum, the part of the brain that controls balance and coordination. A study of long-term users found cerebellar atrophy in about 36% of patients. The strongest predictor wasn’t the daily dose but the total duration of treatment. Patients who had experienced episodes of phenytoin toxicity were hit hardest: 56% of those with a history of toxicity showed moderate to severe shrinkage.
This damage can cause persistent problems with balance, coordination, and fine motor skills that may not fully reverse even after stopping the drug. It’s one reason many neurologists now prefer newer seizure medications for long-term use when possible.
Bone Density Loss
Long-term phenytoin use is linked to decreased bone mineral density, which can progress to osteoporosis and increase fracture risk. The drug speeds up your body’s breakdown of vitamin D by activating liver enzymes that clear it from the bloodstream. Lower vitamin D leads to poor calcium absorption, which in turn triggers your body to pull calcium from your bones.
If you’ve been on phenytoin for years, vitamin D supplementation is worth discussing with your provider, especially if you get limited sun exposure, don’t consume much dietary calcium, or are less physically active. The bone loss develops gradually and often goes undetected until a fracture occurs.
Serious Skin Reactions
Phenytoin can trigger severe, potentially fatal skin reactions including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). These conditions cause widespread blistering and peeling of the skin and mucous membranes and require emergency treatment. Any new rash that appears after starting Dilantin should be evaluated immediately, and the drug should be stopped unless the rash is clearly unrelated.
Genetics play a role in this risk. People of Thai or Han Chinese descent carry a specific gene variant (HLA-B*1502) at rates of about 8 to 9%, and this variant is strongly linked to phenytoin-triggered SJS. Among carriers, the gene has a 33% positive predictive value for developing SJS on phenytoin. In people without the variant, the risk drops to essentially zero. The prevalence of this gene variant is much lower (1 to 2%) in people of European descent.
Other Rare but Serious Reactions
Several organ systems can be affected in uncommon but dangerous ways. The FDA label lists cardiac effects including slow heart rate and cardiac arrest, both at normal doses and during toxicity. Acute liver damage, including rare cases of liver failure, has been reported. Blood cell production can also be disrupted, leading to dangerously low counts of platelets, white blood cells, or other blood components.
A reaction called DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms) can develop, typically presenting with fever, rash, swollen lymph nodes, and facial swelling alongside inflammation of the liver, kidneys, or heart. It can mimic a severe viral infection and is sometimes fatal. Angioedema, or sudden swelling of the face and airway, has also been reported.
Phenytoin also raises the risk of suicidal thoughts and behavior, a warning that applies broadly to the seizure medication class. This risk exists regardless of the condition being treated.
Pregnancy Risks
Phenytoin is known to cause birth defects. Fetal hydantoin syndrome affects 5% to 11% of babies exposed to the drug during pregnancy. Characteristic features include eyes set wider apart than usual, a flattened nasal bridge, cleft lip or palate, a smaller than expected head size, and underdeveloped fingers, toes, and nails. Crossed eyes are also common. Anyone who could become pregnant and is taking Dilantin needs a clear plan with their provider about seizure control and medication alternatives.
Drug Interactions
Phenytoin is one of the most potent drug interaction culprits in medicine. It strongly activates liver enzymes that break down other medications, particularly the CYP3A pathway. This means it can dramatically reduce the effectiveness of many common drugs, including hormonal birth control, blood thinners, certain heart medications, steroids, immunosuppressants, and other seizure drugs. It also moderately affects two other enzyme pathways (CYP1A2 and CYP2C19), broadening its interaction profile even further.
In practical terms, a drug that normally works well may lose more than 80% of its effectiveness when taken alongside phenytoin. This is especially critical for medications with narrow margins of their own, like blood thinners or transplant rejection drugs. Any time a new medication is added or removed from your regimen, phenytoin blood levels and the effectiveness of the other drug both need to be reassessed.
Stopping Dilantin Safely
Abruptly stopping phenytoin can trigger a dangerous prolonged seizure known as status epilepticus. Any dose reduction or discontinuation needs to happen gradually under medical supervision, even if side effects are bothering you. Switching to an alternative medication is typically done by slowly tapering phenytoin while gradually introducing the replacement drug.