Brain cancer is one of the deadliest diseases — and early detection is crucial for better outcomes.

There are obvious symptoms like sudden, severe headaches and dizziness, while subtle signs such as sweet smells and vision changes can slip under the radar.

Now, an international team of researchers is raising alarm bells about another early warning sign that affects millions of people.

About 1 in 10 US adults experience a seizure at some point. A study published Monday in JAMA Neurology found that this abnormal electrical activity in the brain could be one of the first indicators of brain cancer.

Of course, seizures don’t always mean a cancer diagnosis, according to Dr. Randy D’Amico, director of the Brain and Spine Metastasis Program and associate professor of neurosurgery at Northwell Lenox Hill Hospital.

“I think the seizure is the smoke, not necessarily the fire,” D’Amico told The Post. “The fire is probably already ongoing within the brain. You have to treat a first-time seizure as a very serious event.”

The study looked at nearly 49,900 Danish adults who had experienced a seizure at an average age of 50.

First-time seizures were connected to short-term risk of developing cancer within a year, as well as a slightly increased long-term risk.

This link makes sense to D’Amico due to the anatomy of the brain.

“Tumors that are in our brain cortex or swelling from these things can disrupt normal electrical activity,” he explained. “And when that happens, or increased pressure due to the tumor, these things can provoke seizures.”

It’s not just the brain affected by seizures, though.

Another notable finding from the study is that a first seizure as an adult could also be an indicator of non-neurological cancers, with the largest risks including lung, bronchial and tracheal cancers.

D’Amico said this could happen because of how cancer affects the brain, regardless of where the disease is in the body.

“You may not know that you have cancer, but the tumor itself or the cancer that’s developing is causing things like small metastatic disease to the brain,” he said.

“Lung cancer loves to go to the brain,” he added. “A small tumor with swelling can provoke a seizure in the brain.”

While the risk of developing cancer in a short timespan after a seizure is high, D’Amico advises those who have experienced one to approach it with a sense of urgency rather than panic.

Seizures can be caused by a number of factors, such as the ability to control sodium in the body, inflammation, changes in breathing, hypoxia or sugar fluctuations.

“I think the important thing here is that the first seizure may be telling you that something serious is happening,” he said. “And if you miss that window of opportunity, which this study is saying is early, it can delay diagnosis.”