Up to 10 percent of adults will have a seizure at some point in their lives. The good news is that most people who have one seizure don’t go on to develop epilepsy (recurrent seizures).

A seizure happens when the brain’s electrical activity misfires. Normally, neurons (a type of brain and nervous system cell) send signals to each other in an organized way. During a seizure, that pattern becomes abnormal, excessive, or out of sync.

A first-time seizure can be caused by a wide variety of causes. Sometimes there’s no clear cause at all.

Here we’ll cover potential causes for first-time seizures in adults, as well as tests your care team might recommend.

Brain and Nervous System Causes

Some of the most common causes of a first seizure start in the brain. Doctors call these structural causes.

Stroke

Stroke is one of the most common causes of first-time seizures in adults, especially older adults.

A stroke happens when part of the brain doesn’t get enough blood flow. Stokes can cause damage that interferes with the brain’s electrical signals and causes seizures.

The risk of stroke-related seizures rises with age because strokes become more common later in life. A seizure can happen in the days right after a stroke, or it can show up months or years later as scar tissue forms in the injured part of the brain.

Traumatic Brain Injury

Traumatic brain injury (TBI) is a leading cause of new seizures in younger and middle-aged adults.

TBIs can be caused by situations such as a blow to the head, a car accident, or a bad fall. More severe TBI, especially if it causes a loss of consciousness, is a recognized cause of seizures.

Brain Tumors

A brain tumor, whether it’s cancerous or not, can press on brain tissue or disrupt normal electrical signals. This can lead to a seizure. Tumors become a more common cause of first seizures as people get older.

Brain Infections and Inflammation

Infections such as meningitis or encephalitis cause inflammation in or around the brain. This swelling can interfere with normal brain signals and trigger a seizure. These infections often come with fever, a stiff neck, or sudden confusion.

Less commonly, the immune system may mistakenly attack the brain, as it does in some autoimmune diseases, and cause similar inflammation.

Metabolic and Chemical Causes

Not every seizure starts in the brain. Sometimes the trigger is a chemical imbalance somewhere else in the body.

Low Blood Sugar

When blood sugar drops too low, a condition called hypoglycemia, the brain doesn’t get the fuel it needs to function normally. This can trigger a seizure, particularly in people with diabetes who take insulin or other blood sugar-lowering medicines.

Very high blood sugar can also, in rare cases, contribute to seizures.

Electrolyte Imbalances

Sodium, calcium, and magnesium are electrolytes that help nerve cells fire normally. When any of these fall too low, the risk of seizure goes up. Electrolytes can drop this low due to severe dehydration, kidney problems, liver failure, or certain medicines that affect fluid balance.

Low Blood Oxygen and Blood Pressure Extremes

When the brain doesn’t get enough oxygen, even briefly, it can cause a seizure. This may happen during a serious illness, a heart problem, or breathing problems (such as sleep apnea) that lower oxygen levels.

Very high blood pressure can also cause seizures, especially during a pregnancy complication called eclampsia.

Medications and Substances

Several common medications, along with alcohol and recreational drug use, can raise the risk of a first seizure.

Medications That Raise the Risk for Seizures

Certain medications can, in rare cases, lower the seizure threshold, making it easier for a seizure to happen. These include:

• The antidepressant bupropion, sold as Wellbutrin
• Antihistamines (such as diphenhydramine, sold over the counter as Benadryl)
• The pain medicine tramadol

Certain antibiotics and antipsychotic medicines also carry this risk. If you’ve had a seizure for the first time, ask your doctor to review your medication list.

Alcohol and Drug Withdrawal

Suddenly stopping heavy alcohol use, or stopping certain sedative medicines like benzodiazepines, can trigger a seizure during withdrawal. This is most likely in people with long-term heavy use or physical dependence.

Because withdrawal can be dangerous, it’s usually safest to do it under medical supervision.

Stimulant drugs such as cocaine and methamphetamine are also linked to seizures, both during regular use and during an overdose.

Other Possible Causes

A handful of other factors can also lead to a first seizure.

Sleep Deprivation

Going without enough sleep, even for one night, can lower the brain’s seizure threshold. Sleep loss is a recognized trigger for seizures in adults.

Genetic and Unknown Causes

In many cases, no clear cause of seizures is ever found, even after extensive testing. These unprovoked seizures tend to be more common in younger adults, and genetics may play a role in some cases.

Seizure Look-Alikes

A few conditions can look like a seizure without actually being one, which is part of why getting an accurate diagnosis matters.

• Fainting — This can sometimes involve brief shaking or jerking that’s easy to mistake for a seizure.
• Postural orthostatic tachycardia syndrome, or POTS — POTS causes the heart rate to spike when a person stands up. The resulting drop in blood flow to the brain can trigger fainting spells that look like a seizure.
• Severe panic attacks — A racing heart, shaking, and a feeling of disconnection from reality can be mistaken for a seizure.

Some seizurelike episodes don’t involve abnormal electrical activity in the brain at all. These are called psychogenic nonepileptic seizures, or functional seizures, and they happen because of physical or psychological distress.

They can happen alongside true epileptic seizures, so telling the two apart usually requires a careful history and sometimes specialized monitoring.

What Tests Might Your Care Team Recommend?

Finding the cause matters. About 40 percent of first seizures are linked to an identifiable provoking factor, and treating that cause can lower the chance of having another seizure.

Your care team will use everything they learn to decide whether you need ongoing treatment to help prevent another seizure.

Blood and urine tests are often the first step. These can check blood sugar, electrolyte levels, kidney and liver function, and screen for drugs or alcohol that might explain the seizure. A pregnancy test is also standard for people of childbearing age, since pregnancy-related conditions can sometimes trigger seizures.

An electroencephalogram (EEG) records the brain’s electrical activity through small sensors placed on the scalp. It’s painless and can pick up patterns that suggest a higher chance of another seizure. Doctors recommend this test as soon as possible after a first seizure, ideally within the first day.

Brain imaging is also common. A CT scan can quickly rule out bleeding, swelling, or trauma, which makes it especially useful in the emergency room. An MRI, which captures more detail, is often recommended afterward to look for tumors, scarring, or other structural issues a CT scan might miss.

In some cases, a lumbar puncture (spinal tap) may be recommended if your care team suspects an infection in or around the brain. This involves collecting a small sample of the fluid that surrounds the brain and spinal cord for testing.

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