Epilepsy in adults develops from a wide range of causes, but the most common ones are stroke, head injuries, brain tumors, and alcohol use. In roughly 30% to 40% of adult cases, no specific cause is ever identified. When a cause can be found, it generally falls into one of six categories: structural damage to the brain, genetic factors, infections, metabolic disturbances, immune system problems, or unknown origins.
Stroke: The Leading Cause in Older Adults
Stroke is the single most common identified cause of epilepsy in adults over 35. When blood flow to the brain is interrupted (ischemic stroke) or a blood vessel bursts (hemorrhagic stroke), the resulting damage can create scar tissue and abnormal electrical pathways that trigger recurring seizures. Between 2% and 14% of ischemic stroke survivors develop epilepsy afterward, and the risk roughly doubles for hemorrhagic stroke, reaching 10% to 20%.
Timing varies, but most post-stroke epilepsy emerges within the first year. Between 40% and 80% of cases show up in that window. Some people have their first seizure within days of the stroke, while others don’t develop epilepsy until months or even years later. The location and size of the stroke both influence risk, with larger areas of damage and strokes affecting the brain’s outer layers (the cortex) carrying the highest likelihood.
Traumatic Brain Injury
A serious blow to the head can rewire the brain’s electrical activity in ways that produce seizures months or years down the road. The risk scales directly with how severe the injury is. Mild concussions carry relatively low risk on their own, but focal injuries, where a specific area of brain tissue is damaged, carry the highest. In one large registry study, people with focal brain injuries who had a post-traumatic seizure had a 62% chance of being diagnosed with epilepsy within 10 years. For diffuse brain injuries the figure was about 54%, and for bleeding outside the brain tissue itself it was around 51%.
The timing of that first seizure matters too. If a seizure occurs within two years of the injury, the risk of developing ongoing epilepsy is higher than if the first seizure happens later. Mild head injuries and skull fractures without deeper brain damage did not significantly increase epilepsy risk compared to the general population in the same study, which is reassuring for people recovering from minor concussions.
Brain Tumors
Seizures are one of the most common symptoms of brain tumors, and in some cases, a seizure is the very first sign that a tumor exists. The likelihood depends heavily on the tumor type. Slow-growing, low-grade tumors tend to cause seizures more often than fast-growing ones. Certain rare tumor types, such as dysembryoplastic neuroepithelial tumors and gangliogliomas, cause seizures in up to 80% to 100% of cases. These are among the most common tumors found in adults who undergo surgery for drug-resistant focal epilepsy.
Even tumors that have spread to the brain from elsewhere in the body (metastases) cause seizures in about 35% of patients, with the seizure being the first noticeable symptom in roughly one in five. Location plays a major role: tumors near the brain’s surface and in the temporal lobe are more likely to trigger seizures than those seated deeper inside the brain.
Alcohol and Drug Use
Alcohol is one of the most common triggers for seizures in adults, and chronic heavy drinking can eventually cause lasting epilepsy. The mechanism works in two directions. Binge drinking lowers the seizure threshold acutely, and long-term alcohol use physically changes how the brain’s excitatory and inhibitory systems balance each other. Alcohol withdrawal is particularly dangerous because the brain, accustomed to being suppressed by alcohol, rebounds into a hyperexcitable state that can produce severe seizures within 6 to 48 hours of the last drink.
Recreational drugs, prescription medication misuse, and certain toxin exposures can also trigger seizures. When these seizures recur or lead to lasting brain changes, they can cross the line into epilepsy. Epidemiologically, alcohol and drugs are considered the most important causes of acute symptomatic seizures in adulthood.
Autoimmune Epilepsy
The immune system can sometimes mistakenly attack healthy brain tissue, producing inflammation that leads to seizures. This is called autoimmune encephalitis, and it’s an increasingly recognized cause of new-onset epilepsy in adults. The body produces antibodies that target proteins on the surface of brain cells or inside neurons, disrupting normal signaling.
Diagnosis typically involves blood and spinal fluid tests to check for specific antibodies, brain MRI, PET scans that measure how well the brain is processing nutrients, and EEG recordings to detect seizure activity. Some antibody types respond well to immune-targeted treatments, but others are harder to control. People with one particular antibody (GAD65) tend to have a harder time becoming seizure-free. Autoimmune epilepsy is worth investigating when seizures start suddenly in an otherwise healthy adult, especially if they come alongside confusion, personality changes, or memory problems.
Neurodegenerative Diseases
Alzheimer’s disease and other forms of dementia significantly raise the risk of developing epilepsy later in life. Among people with Alzheimer’s, 10% to 22% experience at least one unprovoked seizure. When researchers use prolonged brain wave monitoring during sleep, they find abnormal electrical activity in 30% to 50% of Alzheimer’s patients, meaning many seizures likely go undetected.
The connection runs through the same proteins that drive Alzheimer’s itself. Amyloid plaques and abnormal tau protein can push neurons into a hyperexcitable state, increasing the rate at which they fire and disrupting the channels that normally keep electrical activity in check. Early in the disease, this hyperexcitability may actually accelerate cognitive decline, creating a vicious cycle where seizures and dementia worsen each other. This is an area where treatment could potentially slow both problems, though the relationship is complex and changes as the disease progresses.
Genetic Causes in Adults
Epilepsy is often thought of as a childhood genetic condition, but genetic causes can surface for the first time in adulthood. Some people carry mutations that produce mild or intermittent seizures for years before anyone recognizes the pattern as epilepsy. Others have genetic conditions diagnosed in childhood that persist into adult life, often without ever receiving a definitive genetic diagnosis.
One well-characterized example is autosomal dominant lateral temporal lobe epilepsy, which typically begins in adolescence or early adulthood. It causes focal seizures in people who otherwise have normal intelligence and normal-looking brain scans. Variants in a gene called LGI1 account for most familial cases. Other genes linked to adult epilepsy include SCN1A (the most commonly affected gene in certain severe epilepsy types), STXBP1, and CHD2. Genetic testing is becoming more accessible, and identifying a specific mutation can sometimes guide treatment choices or explain why standard medications haven’t worked.
Metabolic Disturbances
The brain depends on a tightly controlled chemical environment. When that balance is disrupted, seizures can follow. Common metabolic triggers include severely low blood sugar, low sodium levels, kidney failure (uremia), and liver failure. These conditions typically cause acute seizures that resolve once the underlying problem is corrected, but repeated or prolonged metabolic crises can sometimes cause lasting damage that leads to chronic epilepsy.
Kidney patients on dialysis face a specific risk called dialysis disequilibrium syndrome, where rapid fluid shifts during treatment cause brain swelling and can trigger seizures. Endocrine disorders, particularly those affecting thyroid or parathyroid function, can also lower the seizure threshold. In rare cases, a treatable vitamin deficiency (pyridoxine, or vitamin B6) causes seizures that don’t respond to standard anti-seizure medications but resolve completely with the right supplement. Early identification of these metabolic causes matters because the treatment is fundamentally different from conventional epilepsy management.
Infections That Affect the Brain
Any infection that reaches the brain or its surrounding membranes can cause seizures. Bacterial meningitis, viral encephalitis (including herpes simplex), and parasitic infections like neurocysticercosis are well-established causes. Neurocysticercosis, caused by a pork tapeworm larva lodging in brain tissue, is actually the most common cause of adult-onset epilepsy worldwide, particularly in regions of Latin America, sub-Saharan Africa, and South and Southeast Asia.
HIV also raises epilepsy risk, both through direct effects on the brain and through opportunistic infections that take hold when the immune system is weakened. In most infectious cases, the seizures result from inflammation and scarring left behind after the infection clears. This means epilepsy can develop weeks, months, or even years after the original illness, long after the infection itself has been treated.
When No Cause Is Found
Despite thorough testing, a significant portion of adult epilepsy cases remain unexplained. This doesn’t mean there is no cause. It usually means current technology can’t detect the underlying problem. The brain change may be too subtle for standard MRI, or it may involve a genetic variant that hasn’t been linked to epilepsy yet. As imaging and genetic testing improve, some of these “unknown” cases are being reclassified. For now, the good news is that epilepsy without an identified cause often responds well to medication, and many people in this category achieve full seizure control.