Poisoning Due to Anticonvulsants (Seizure Medicine Overdose)

What to do right now

1. Call 911 if the person is unconscious, having a seizure, struggling to breathe or the overdose was intentional.
2. Otherwise call Poison Control (1-800-222-1222 in the US, available 24 hours) for advice on whether hospital care is needed.
3. Do not make the person vomit and do not give anything to eat or drink.
4. If they are drowsy but breathing, place them on their side in the recovery position and stay with them.
5. If a seizure happens, protect the head, do not put anything in the mouth, time the seizure, and turn them on their side afterward.
6. Bring the medicine containers to the hospital, including any extended-release products, and note the time and amount taken.

How poisoning happens

• Intentional overdose: often involves several drugs together.
• Accidental ingestion by young children: some tablets, especially carbamazepine and lamotrigine, can be dangerous in small amounts.
• Gradual build-up (chronic toxicity): common with phenytoin and carbamazepine, whose levels can rise sharply after small dose increases, illness, or drug interactions.
• Drug interactions: for example, valproate raises lamotrigine and phenobarbital levels; some antibiotics (clarithromycin, erythromycin) and antifungals raise carbamazepine levels.
• Kidney or liver problems: reduce clearance of drugs such as gabapentin, pregabalin and levetiracetam.

Signs by medicine

Nystagmus (jerky, flickering eye movements, especially looking sideways), ataxia (unsteady walking) and slurred speech together are a classic early pattern of toxicity, particularly with phenytoin and carbamazepine. See nystagmus and ataxia.

How it is diagnosed

• History: which medicine, how much, when, immediate or extended-release, other drugs or alcohol.
• Examination: level of consciousness, breathing, pupils, eye movements, coordination.
• Blood drug levels: available for carbamazepine, phenytoin, valproate, phenobarbital and some others. Levels are repeated because some keep rising for hours.
• ECG: looks for wide QRS, prolonged PR or QT, and heart rhythm problems.
• Other blood tests: glucose, sodium, kidney and liver function, ammonia (valproate), blood gas for acidosis, albumin (low albumin increases free phenytoin), and screening for acetaminophen (paracetamol) and aspirin in any intentional overdose.

Hospital treatment

Supportive care

Most people recover with monitoring and support: oxygen, IV fluids, protecting the airway, and a breathing machine if needed. Seizures are treated with benzodiazepines.

Decontamination

Activated charcoal may be given if the person is awake, protecting their airway and arrives early. Multiple doses of charcoal are used for carbamazepine and phenobarbital to speed removal from the gut and blood.

Specific treatments

• Sodium bicarbonate IV for wide QRS from carbamazepine or lamotrigine.
• L-carnitine for valproate poisoning with high ammonia or liver toxicity.
• Hemodialysis for severe carbamazepine, phenobarbital or valproate poisoning, particularly with coma, shock or very high levels, following EXTRIP recommendations.
• Stopping the drug and restarting at a safer dose for chronic phenytoin or carbamazepine toxicity.

After recovery

Everyone with an intentional overdose should have a mental health assessment. For people with epilepsy, doctors review the seizure plan, because seizures can return if the medicine is stopped abruptly or levels fall too low.

Outlook

Most people recover fully, particularly from chronic toxicity and from overdoses of levetiracetam, gabapentin and similar drugs. Large overdoses of carbamazepine, valproate, phenobarbital and lamotrigine can be life-threatening but are usually survivable with intensive care. Complications include aspiration pneumonia, brain injury from prolonged low oxygen, and liver failure with valproate.

Preventing poisoning

• Keep all medicines locked away from children and in original packaging.
• Use a pill organizer, and never double up after a missed dose unless told to.
• Ask your pharmacist to check interactions whenever another medicine is started or stopped.
• Have levels checked after dose changes when your doctor advises it.
• If you live with someone at risk of self-harm, limit the amount of medicine kept at home.

Frequently asked questions

What are the symptoms of anticonvulsant toxicity?

Early symptoms are drowsiness, dizziness, unsteady walking, slurred speech, double vision and jerky eye movements. Nausea and vomiting are common. Severe toxicity causes confusion, seizures, slow breathing, irregular heart rhythm and coma.

What phenytoin level is toxic?

The usual target total phenytoin level is about 10 to 20 mcg/mL. Nystagmus often appears above 20, unsteadiness above about 30, and confusion and drowsiness above about 40. Low blood albumin, kidney failure or interactions can cause toxicity even when the total level looks normal.

Can you overdose on gabapentin?

Yes. Gabapentin and pregabalin overdoses usually cause drowsiness, dizziness and slurred speech and are rarely fatal on their own. They become dangerous when combined with opioids, alcohol or sedatives, because together they can stop breathing.

Why can carbamazepine overdose get worse after a few hours?

Carbamazepine is absorbed slowly and unevenly, and large amounts can clump in the stomach and intestines. Levels may keep rising or rise again after an initial improvement. This is why people are monitored with repeated blood levels and may receive multiple doses of activated charcoal.

Should I stop my seizure medicine if I feel dizzy?

Do not stop seizure medicine suddenly, because this can trigger seizures. If you feel dizzy, unsteady or very drowsy, contact your doctor or pharmacist the same day for advice and possibly a blood level test. Seek emergency care for severe symptoms.