What should I do if I accidentally take an extra 100 mg of oral phenytoin (Dilantin)?

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Last updated: September 29, 2026 • View editorial policy

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What to Do If You Take an Extra 100 mg of Phenytoin

If you accidentally take an extra 100 mg of oral phenytoin (Dilantin), contact your healthcare provider or poison control immediately to report the overdose, even if you feel fine, as symptoms may develop over hours and the drug has a prolonged elimination time. 1

Immediate Actions

  • Call your healthcare provider or poison control right away to report the accidental extra dose
  • Do not try to induce vomiting or take additional medications without medical guidance
  • Monitor yourself closely for symptoms of toxicity over the next several hours to days

Why This Matters

Phenytoin has zero-order (saturable) pharmacokinetics, meaning that even a modest overdose can lead to disproportionately high blood levels that persist for days to over a week 2, 3. A single extra 100 mg dose may push you into toxic range, especially if you're already taking regular doses.

Symptoms to Watch For

The FDA label warns you should seek immediate medical attention if you develop 1:

Central nervous system effects (most common):

  • Irregular eye movements (nystagmus)
  • Problems with balance and coordination (ataxia)
  • Slurred speech
  • Drowsiness or confusion
  • Dizziness

Cardiac effects:

  • Feeling like your heart is beating slowly or skipping beats
  • Chest pain
  • Unusual tiredness

Serious allergic reactions (rare but dangerous):

  • Fever, rash, swollen lymph glands
  • Swelling of face, lips, or tongue
  • Difficulty breathing or swallowing

What Healthcare Providers Will Do

Supportive care is the only treatment - there is no antidote for phenytoin overdose 2, 3. Your provider may:

  • Check your phenytoin blood level
  • Monitor you for symptoms
  • Provide supportive care if symptoms develop (managing nausea, preventing falls from ataxia)
  • In severe cases with very high levels, consider activated charcoal if you present early (within 1-2 hours), though evidence for benefit is limited 2, 3

Important Caveats

  • Do not skip your next scheduled dose without talking to your provider - abruptly stopping phenytoin can cause serious seizures 1
  • Symptoms may be delayed and can persist for days due to the drug's prolonged half-life in overdose 2, 3
  • Deaths from oral phenytoin overdose alone are extremely rare, even with massive ingestions 2, 4
  • Enhanced elimination methods (dialysis, hemoperfusion) are generally not beneficial except in extreme cases 2, 5

The key is early notification of your healthcare provider so they can guide monitoring and management based on your specific situation, baseline dose, and any symptoms that develop.

References

2
Research

Phenytoin poisoning.

Neurocritical care, 2005

3
Research

Clinical features and management of poisoning due to phenytoin.

Medical toxicology and adverse drug experience, 1989

4
Research

Presentations of acute phenytoin overdose.

The American journal of emergency medicine, 1989

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.