A seizure that strikes out of nowhere usually has a identifiable trigger, even when it feels completely random. The most common causes include a sharp drop in blood sugar, a sudden shift in blood sodium levels, alcohol withdrawal, a stroke, a head injury, an infection reaching the brain, or a medication side effect. About half the time, doctors can pinpoint exactly what provoked a first seizure. The other half are classified as “unprovoked,” meaning no immediate trigger is found, though an underlying cause may still exist.
Blood Sugar and Electrolyte Shifts
Your brain is extremely sensitive to changes in blood chemistry. When blood sugar drops below about 54 mg/dL, the brain loses its primary fuel source, and seizures become a real possibility. This can happen to people with diabetes who take too much insulin, skip meals, or exercise more than usual. It can also occur in people without diabetes who go long periods without eating, especially if they’re also drinking alcohol.
Sodium is the other major player. Your body keeps blood sodium in a tight range, and when it falls below 125 mEq/L, water shifts rapidly into brain tissue, causing swelling. That swelling can trigger seizures, confusion, and in rare cases, life-threatening brain compression. Sodium can drop this low from excessive water intake, certain medications (especially some antidepressants and diuretics), severe vomiting or diarrhea, or kidney problems. The speed of the drop matters as much as the number itself: a gradual decline over days may cause only mild confusion, while a rapid plunge over hours is far more likely to cause a seizure.
Alcohol and Drug Withdrawal
If someone who drinks heavily stops suddenly, the brain essentially rebounds from being chronically sedated. Alcohol suppresses brain activity, and when it’s abruptly removed, nerve cells fire in an uncontrolled burst. Withdrawal seizures typically hit between 6 and 48 hours after the last drink, with the highest risk window around 12 to 48 hours. About 1% of people going through alcohol withdrawal experience a seizure, and it can be the very first sign that withdrawal is happening.
Benzodiazepines (anti-anxiety medications like diazepam or lorazepam) carry a similar risk when stopped abruptly after long-term use. The mechanism is nearly identical: the brain has adapted to constant sedation and overreacts when the drug disappears. This is why these medications should always be tapered gradually under medical supervision rather than stopped cold.
Stroke and Blood Vessel Problems
A seizure can be the first and only visible sign of a stroke, particularly in older adults. In a study of over 2,300 stroke patients, about 2.3% had a seizure as their presenting symptom, before anyone knew a stroke was happening. The seizure occurs because a blood vessel in the brain either ruptures or gets blocked, cutting off oxygen to a region of brain tissue. That oxygen-starved tissue becomes electrically unstable and fires chaotically.
This is one reason a first-time seizure in someone over 50 is treated urgently. A brain bleed from a ruptured aneurysm or a clot blocking a major artery can look like “just a seizure” on the surface while something far more dangerous is unfolding underneath. Other vascular causes include severely high blood pressure (hypertensive crisis) and blood clots in the brain’s venous drainage system, which are more common in younger women, especially those on hormonal birth control.
Infections That Reach the Brain
Meningitis and encephalitis, infections of the membranes surrounding the brain or the brain tissue itself, are potent seizure triggers. In bacterial meningitis specifically, seizures occur in about 17% of cases. Some of those seizures happen before the person even reaches the hospital, sometimes as the very first symptom.
The infection doesn’t always start in the brain. A urinary tract infection, pneumonia, or other systemic infection can provoke a seizure in someone whose seizure threshold is already low, particularly in older adults or people with a history of brain injury. High fevers from any cause can also push the brain past its threshold, though fever-related seizures are far more common in young children than adults.
Medications That Lower the Seizure Threshold
Several common prescription drugs can make the brain more prone to seizing. The classes most frequently involved include:
- Certain antidepressants: Tricyclic antidepressants carry the most recognized risk, with seizures most likely to appear within the first two to six weeks of starting the medication. SSRIs and other newer antidepressants carry a lower but real risk.
- Some antibiotics: High-dose penicillins given intravenously are a relatively common trigger in hospital settings. Certain other antibiotics used for serious infections also carry this risk.
- Pain medications: One opioid pain reliever, pethidine (meperidine), is the single most common seizure-provoking drug identified in clinical practice.
- Stimulants and recreational drugs: Cocaine, amphetamines, and synthetic stimulants can all trigger seizures, sometimes on first use.
Paradoxically, even anti-seizure medications themselves can occasionally worsen seizures if the dose climbs too high or if the wrong type is prescribed for a particular seizure pattern.
Head Injury and Brain Tumors
A blow to the head can cause a seizure immediately at the time of impact, or days to weeks later as the brain swells or bleeds internally. Post-traumatic seizures are especially common after penetrating injuries, skull fractures, or any trauma that causes bleeding inside the skull. Even a concussion that seemed minor at the time can, in rare cases, lower the seizure threshold enough to cause a seizure days afterward.
Brain tumors, whether cancerous or benign, can irritate surrounding tissue and trigger seizures. In fact, a seizure is sometimes the first clue that a tumor exists. The seizure pattern often depends on where the tumor sits: a tumor near the motor cortex might cause rhythmic jerking of one arm, while a tumor in the temporal lobe might cause a brief episode of staring, confusion, or strange sensory experiences.
Sleep Deprivation and Other Overlooked Triggers
Severe sleep deprivation is one of the most underestimated seizure triggers. It lowers the brain’s threshold for abnormal electrical activity, which is why EEG tests sometimes deliberately use sleep deprivation to provoke detectable brain wave abnormalities. People who work night shifts, new parents, or anyone going through a stretch of very poor sleep may be more vulnerable than they realize, especially if other risk factors are present.
Extreme physical exhaustion, dehydration, and flashing or flickering lights (in people with photosensitive brains) are other triggers that can push someone over the edge without any underlying disease.
What Happens After a First Seizure
A first-time seizure typically leads to blood tests checking for metabolic problems like low sodium, low blood sugar, kidney failure, or liver dysfunction. No single blood test can confirm that a seizure happened, but these tests help identify what caused it. If there are any red flags, such as a fever, head trauma, a neurological deficit like weakness on one side, or a history of cancer, a CT scan of the brain is done immediately to look for bleeding, swelling, or a mass.
When no obvious provocation is found, an MRI using a specialized epilepsy protocol gives a much more detailed look at brain structure. An EEG, which measures electrical activity across the brain’s surface, should ideally be done within 24 hours of the seizure, when abnormal patterns are most likely to still be detectable. If the first EEG is normal, a follow-up study, often done while sleep-deprived, is typically recommended within a week.
Does a Single Seizure Mean Epilepsy?
Not necessarily. Epilepsy is defined as either two or more unprovoked seizures occurring more than 24 hours apart, or a single unprovoked seizure combined with brain imaging or EEG findings that suggest a high risk of recurrence. A seizure with a clear, reversible trigger, like dangerously low blood sugar or alcohol withdrawal, is classified as “provoked” and does not count toward an epilepsy diagnosis on its own.
After a single unprovoked seizure, the risk of having a second one is about 32% within the first year and 46% within five years. Most recurrences happen in the first two years. That means the majority of people who have one unprovoked seizure will not have another, but the odds are high enough that doctors take it seriously and monitor closely. Whether to start daily anti-seizure medication after a single event depends on individual risk factors, including what the EEG and MRI show, and is a decision made case by case.