Dilantin Side Effects: What to Expect and When to Call Your Doctor

Dilantin (phenytoin) has been used to treat epilepsy since 1939, making it one of the most well-studied antiepileptic drugs. Like all medications, it carries a range of potential side effects — from common and manageable to rare but serious. Understanding what to watch for helps you stay safe and know when to seek medical care.

Common Side Effects of Dilantin (Related to Blood Levels)

Many of phenytoin's most common side effects are dose-related — meaning they occur more often when your blood level is too high (above 20 mcg/mL). The classic triad of dose-related toxicity includes:

Nystagmus (involuntary eye movements): Your eyes may drift horizontally or move rapidly in different directions. This is often the earliest sign that your phenytoin level is too high. Appears typically when levels exceed 20 mcg/mL.

Ataxia (coordination problems): Unsteady gait, difficulty with balance, or problems with fine motor skills. Typically appears when levels exceed 30 mcg/mL.

Slurred speech (dysarthria): Speech may become slow, thick, or unclear. Often accompanies ataxia when levels are elevated.

Drowsiness and sedation: Feeling excessively tired or mentally foggy, especially when starting or adjusting doses.

Nausea and vomiting: More common when levels are too high, but can occur early in treatment. Taking phenytoin with food can help.

Confusion and mental slowing: Difficulty thinking clearly, forgetfulness, or mental cloudiness.

If you experience any of these symptoms, contact your doctor. They may want to check your phenytoin blood level and adjust your dose.

Long-Term Side Effects of Dilantin

With long-term use, phenytoin can cause some distinctive effects:

Gingival hyperplasia (gum overgrowth): A notable and unique side effect of phenytoin — the gums grow and thicken over the teeth. It affects up to 50% of patients on long-term therapy. Brushing and flossing twice daily and regular dental cleanings can help minimize this effect.

Hirsutism: Increased body and facial hair, which can be distressing, particularly for women.

Coarsening of facial features: Broadening of the nose and lips and thickening of the skin can occur with long-term use.

Decreased bone density: Phenytoin interferes with vitamin D metabolism, leading to decreased calcium absorption and potential osteomalacia or osteoporosis with long-term use. Calcium and vitamin D supplementation and periodic bone density monitoring may be recommended.

Folic acid deficiency: Phenytoin can reduce folic acid levels. This is especially important in women who are pregnant or planning to become pregnant, as low folate increases the risk of neural tube defects.

Serious Side Effects: Call Your Doctor or Go to the ER

Some side effects require immediate medical attention:

Skin rash: Stop Dilantin and call your doctor immediately if you develop any skin rash. Even a mild rash can be an early sign of Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), or DRESS syndrome — all life-threatening. Do NOT resume Dilantin if a rash occurs without explicit medical guidance.

Fever and flu-like symptoms with rash or swollen lymph nodes: These can be signs of DRESS syndrome — a serious allergic reaction affecting multiple organs including liver, kidneys, and heart.

Yellowing of skin or eyes (jaundice), dark urine, or stomach pain: Possible signs of liver damage. Seek medical attention right away.

Unusual bleeding or bruising, persistent sore throat, fever: Signs of possible blood disorders (low white blood cells or platelets).

Chest pain, dizziness, or feeling faint: Phenytoin can rarely cause slow heart rate (bradycardia) or cardiac arrest at toxic levels. Call 911 immediately.

Thoughts of suicide or significant mood changes: All antiepileptic drugs carry an FDA warning for increased risk of suicidal thoughts or behavior. Contact your doctor if you notice new or worsening depression, anxiety, or unusual mood changes.

Side Effects in Special Populations

Pregnancy: Phenytoin is a known teratogen. It increases the risk of orofacial clefts, cardiac defects, and fetal hydantoin syndrome. If you become pregnant while taking Dilantin, contact your doctor immediately — do NOT stop taking it without guidance, but your doctor will carefully weigh the risks.

Southeast Asian patients: Patients with the HLA-B*1502 gene variant — more common in Han Chinese, Thai, Filipino, and Malaysian populations — have a significantly higher risk of SJS/TEN with phenytoin. Genetic testing is recommended before starting the drug in this population.

Tips for Managing Side Effects

Take phenytoin at the same time every day to maintain stable blood levels

Do not take antacids within 2–3 hours of your dose, as they can reduce absorption

Practice good oral hygiene: brush and floss twice daily and see your dentist regularly to manage gingival hyperplasia

Ask your doctor about vitamin D and calcium supplementation if you are on long-term therapy

Keep all scheduled blood monitoring appointments — regular phenytoin levels, CBC, and liver function tests help catch problems early

For a related guide, see Dilantin Drug Interactions: What to Avoid and What to Tell Your Doctor.