A first-time seizure in an adult with no prior history is more common than most people realize, and it almost always has an identifiable trigger or underlying cause. Stroke is the single most common cause, responsible for roughly 23% of new adult-onset seizures, followed closely by cases where no clear cause is found (22%) and central nervous system infections (21%). The rest trace back to metabolic imbalances, brain tumors, head injuries, medications, substance use, and a handful of other conditions.
Understanding what caused a first seizure matters because it shapes everything that comes next: whether treatment is needed, whether another seizure is likely, and what you can do to prevent one.
Stroke and Cerebrovascular Disease
Stroke is the leading identified cause of new seizures in adults, and the connection is especially strong for focal seizures, the type that starts in one specific area of the brain. Among people whose first seizure is focal, stroke accounts for over 40% of cases. Both ischemic strokes (caused by a blood clot) and hemorrhagic strokes (caused by bleeding) can damage brain tissue in ways that disrupt normal electrical activity. Sometimes the seizure happens within days of the stroke. In other cases, scar tissue from an old stroke triggers a seizure months or years later, even if the person recovered well and never had neurological symptoms in between.
Brain Tumors and Structural Lesions
Brain tumors cause about 8% of new adult-onset seizures overall, but that number climbs to nearly 15% when looking only at focal seizures. Both cancerous and noncancerous growths can irritate surrounding brain tissue enough to spark abnormal electrical firing. A seizure is sometimes the very first sign that a tumor exists, particularly slow-growing tumors that haven’t caused headaches or other symptoms yet. Other structural problems, like areas of brain softening from old injuries or blood vessel abnormalities, account for an additional 7% of cases.
Metabolic and Electrolyte Imbalances
About 12% of new adult seizures stem from metabolic disruptions, and these are among the most treatable causes because correcting the imbalance often resolves the problem entirely.
Low sodium (hyponatremia) is the most common electrolyte trigger. Sodium normally sits between 135 and 145 mEq/L in the blood. When it drops rapidly below 115 mEq/L, seizures become a real risk. This can happen from excessive water intake, certain medications (particularly some antidepressants and diuretics), severe vomiting or diarrhea, or hormonal conditions that cause the body to retain too much water.
Low calcium is another frequent culprit. Seizures occur in 20 to 25% of people with acute drops in blood calcium, and the rate is even higher in people with underactive parathyroid glands. Low magnesium rounds out the trio of electrolyte problems most likely to provoke seizures, and it often occurs alongside low calcium since the two minerals are metabolically linked. Extremely low or high blood sugar can also trigger seizures, which is why blood work is one of the first things checked after a first-time event.
Infections of the Brain
Central nervous system infections, including meningitis and encephalitis, account for about 21% of new seizures in adults. Bacterial meningitis, viral encephalitis (particularly from herpes simplex virus), and parasitic infections can all inflame brain tissue enough to cause seizure activity. In many parts of the world, parasitic infections like neurocysticercosis are a leading cause. Fever alone doesn’t typically cause seizures in adults the way it does in young children, but the combination of infection, inflammation, and fever creates a potent trigger.
Autoimmune Encephalitis
A less obvious but increasingly recognized cause is autoimmune encephalitis, where the immune system mistakenly attacks the brain. This condition typically develops over a few weeks to three months and can strike previously healthy adults. About 70% of patients first notice flu-like symptoms: headache, fever, general malaise. Then things progress to memory problems, confusion, personality changes, hallucinations, or seizures.
One well-known form involves antibodies targeting NMDA receptors in the brain, sometimes triggered by a preceding herpes simplex infection. Because the early symptoms can mimic psychiatric illness or a viral infection, autoimmune encephalitis is frequently misdiagnosed at first. It’s treatable when caught, which makes it an important cause to identify.
Alcohol and Substance Withdrawal
Abruptly stopping heavy alcohol use is a well-established seizure trigger. Alcohol withdrawal seizures typically strike 6 to 48 hours after the last drink and are usually generalized tonic-clonic (the type involving full-body convulsions). The brain adapts to chronic alcohol exposure by increasing its excitatory activity to compensate for alcohol’s sedating effect. When the alcohol is suddenly removed, that excess excitatory activity has nothing to counterbalance it.
Recreational drugs can also cause seizures directly. Cocaine and methamphetamine are common triggers. On the prescription side, abruptly stopping benzodiazepines or certain anti-anxiety medications can provoke withdrawal seizures through a similar mechanism to alcohol.
Medications That Lower the Seizure Threshold
Several common prescription and over-the-counter medications can make the brain more susceptible to seizures. Antidepressants are the most frequently reported class, with bupropion leading the list in U.S. data. The pain medication tramadol is another significant cause, particularly in overdose. Older antihistamines like diphenhydramine (the active ingredient in many sleep aids and allergy medications) also carry seizure risk at high doses.
The antibiotic isoniazid, used for tuberculosis, and certain cephalosporin antibiotics can provoke seizures by interfering with GABA, the brain’s primary calming chemical. Stimulant medications, when misused or taken at high doses, round out the common offenders. In most cases, the seizure risk comes from overdose or rapid dose changes rather than normal therapeutic use.
Sleep Deprivation and Stress
Severe sleep loss is a well-documented seizure trigger, so much so that sleep deprivation is deliberately used as a diagnostic tool in epilepsy testing to provoke detectable brain wave abnormalities. The mechanism appears to involve GABA, the same calming brain chemical affected by alcohol and certain medications. During prolonged sleep deprivation, the brain produces fewer GABA receptors responsible for a type of ongoing, background inhibition that normally prevents neurons from firing too rapidly. Without that brake, the brain becomes more excitable.
Extreme physical or emotional stress likely contributes through similar pathways, though the evidence is less precise. People with epilepsy consistently report sleep deprivation and stress as the most common triggers preceding their seizures. For someone with an undetected predisposition, a stretch of very poor sleep, say from shift work, jet lag, or a family crisis, could be the factor that tips the balance.
When No Cause Is Found
In roughly 22% of cases, no specific cause is identified even after thorough testing. These are classified as “unprovoked” seizures. Some of these likely reflect a genetic predisposition to epilepsy that simply manifests later in life. Others may involve subtle brain changes too small to detect on current imaging. An unprovoked first seizure doesn’t automatically mean epilepsy. The recurrence risk after a single unprovoked seizure is estimated at about 34% over five years, though that number ranges widely, from 23% to 80%, depending on individual factors like whether the EEG shows abnormal activity.
What Happens After a First Seizure
After a first-time seizure, doctors typically order two key tests. An EEG (electroencephalogram) records the brain’s electrical activity and is most useful when performed within 24 hours of the event, when abnormal patterns are most likely to show up. An MRI using a specialized epilepsy protocol looks for structural causes: tumors, stroke damage, scarring, blood vessel abnormalities. If there are red flags like fever, a focal seizure, persistent headache, recent head trauma, or a weakened immune system, a CT scan may be done immediately in the emergency department before the MRI.
Blood tests check for the metabolic and electrolyte problems described above. A toxicology screen may be run to identify drug-related causes. If infection or autoimmune disease is suspected, a lumbar puncture (spinal tap) may be needed to analyze spinal fluid.
What to Do During a Seizure
If you witness someone having a seizure for the first time, call 911. A first-ever seizure always warrants emergency evaluation. While waiting for help, keep the person safe by easing them to the ground if they’re falling and clearing the area around them of hard or sharp objects. Turn them gently onto their side with their mouth pointing toward the ground to keep their airway clear. Place something soft under their head.
Time the seizure. If it lasts longer than five minutes, that’s a medical emergency requiring immediate intervention. Equally important is what not to do: don’t hold them down, don’t put anything in their mouth, and don’t offer food or water until they’re fully alert. Most seizures end on their own within one to three minutes. Afterward, the person will likely be confused and disoriented. Stay with them, explain calmly what happened, and help them get to medical care.