Like nearly all conditions that revolve around pain, headaches can be tricky to categorize. A headache one person would call “terrible” might feel mild to someone else.

But speaking generally, you need to see your primary care physician if your headache status changes, says Mark Morocco, M.D., a clinical professor and doctor at the University of California, Los Angeles Medical Center. That means if you never had headaches, but now you seem to have them all the time, your doctor should know about that change, Dr. Morocco says. If the intensity of your regular migraines has suddenly ratcheted up, that’s also something worthy of your doctor’s attention.

Even in those situations, chances are good your headaches are not of the life-threatening variety.

“People are always worried about brain tumors,” Dr. Morocco says. But headaches are actually not among the symptoms experts usually associate with a tumor.  

On the other hand, there are some warning signs that your headache is a true medical emergency. Here’s what to watch out for.

YOU HAVE A ‘THUNDERCLAP’ HEADACHE

If a severe headache comes on suddenly—“Like someone flipping a light switch, or hitting you with a hammer,” Dr. Morocco says—that’s something to take seriously. It could be a “subarachnoid hemorrhage” (SAH), or bleeding within your brain caused by a leaking aneurysm .

“That’s a dangerous headache, and you need to get to a doctor or have someone take you to the hospital,” Dr. Morocco says.

To be clear, this isn’t a sharp pain that recedes in a matter of seconds or minutes. “It won't go away quickly,” he adds, “but you don’t want to make the mistake of taking heavy pain pills and going to sleep.”

YOUR HEADACHE CHANGES ITS TUNE

If you experience migraines or tension headaches, an especially severe headache—while terrible—isn’t necessarily something you should worry about.

“One way to think about it is, if the quality of your headaches were a song, is this the same song, but with the volume turned up higher?” Dr. Morocco says.

If the answer is yes, he says that’s reassuring. What he worries about is if the song changes, meaning your headache feels both severe and different from what you normally experience. If that’s the case, you should get to the hospital, he says. Again, this could be an aneurysm or some other urgent medical situation.

YOU HAVE A ‘THUNDERCLAP’ HEADACHE

If a severe headache comes on suddenly—“Like someone flipping a light switch, or hitting you with a hammer,” Dr. Morocco says—that’s something to take seriously. It could be a “subarachnoid hemorrhage” (SAH), or bleeding within your brain caused by a leaking aneurysm. 

“That’s a dangerous headache, and you need to get to a doctor or have someone take you to the hospital,” Dr. Morocco says.

To be clear, this isn’t a sharp pain that recedes in a matter of seconds or minutes. “It won't go away quickly,” he adds, “but you don’t want to make the mistake of taking heavy pain pills and going to sleep.”

YOUR HEADACHE CHANGES ITS TUNE

If you experience migraines or tension headaches, an especially severe headache—while terrible—isn’t necessarily something you should worry about.

“One way to think about it is, if the quality of your headaches were a song, is this the same song but with the volume turned up higher?” Dr. Morocco says.

If the answer is yes, he says that’s reassuring. What he worries about is if the song changes, meaning your headache feels both severe and different from what you normally experience. If that’s the case, you should get to the hospital, he says. Again, this could be an aneurysm or some other urgent medical situation.

YOUR HEADACHE ISN'T YOUR ONLY ISSUE

A bad headache accompanied by a fever is concerning.

“This could indicate an infection of the brain—something like meningitis,” Dr. Morocco says.

It could also be a warning sign of encephalitis, or inflammation of the brain. And if your bad headache and fever are accompanied by an altered mental state—you can’t remember your kids, or are otherwise acting unlike yourself—that’s a good reason to go to the hospital.

THE ACHE IS BEHIND OR AROUND YOUR EYE

“If you have a headache with eye pain, and especially if you have a change in your vision, that’s an emergency,” Dr. Morocco says.

The big concern here is acute glaucoma, or a buildup of pressure in your eye that cuts off its blood flow and can result in blindness.

“What we see a lot is a person goes to a movie, and when the lights go down and the pupil dilates, that change in pressure leads to the headache and the other symptoms,” he says.

THE PAIN IS CONCENTRATED IN YOUR TEMPLE

In most cases, the placement of your headache isn’t an indication of anything scary, Dr. Morocco says. But if you’re older than 50 and your headache feels tightly focused in one or both of your temples, that could be a sign of temporal arteritis—a condition in which the arteries in your temples become inflamed. Especially if you have blurred vision or a fever, you want to see a doctor immediately.

“It can result in loss of vision if we don’t treat it,” he adds.

YOU BUMPED YOUR HEAD—AND YOU’RE ON BLOOD THINNERS

If you knock your noggin and are on blood thinners, a headache could indicate a subdural hematoma, or a kind of slow bleed inside your brain that—thanks to those thinners—doesn’t clot, Dr. Morocco says.

This situation can be deadly, he adds, so get to a hospital. 

YOUR HEADACHE IS CONTAGIOUS

If people around you—your family, maybe, or coworkers—are complaining about their heads at the same time you’re experiencing an unusual ache, that could be a sign of C02 poisoning, Dr. Morocco says.

If you step outside and your headache lightens, warn everyone else, open windows or doors, and have the space inspected for a CO2 leak.