Dangerous headaches also tend to have some distinctive features and patterns – and they can be hard to ignore. Here's an overview of those red flag symptoms, and when you need same-day or emergency care:

The worst headache, in no time flat

You may have been told to be wary of a sudden headache, but just how sudden does it have to be to be a red flag? It's a lot quicker than you'd think.

We're talking in terms of seconds – the headache comes on fast with an intensity that stops you in your tracks, feels downright unbearable, and the pain peaks within less than a minute or so (although the headache may continue for longer). This is called a thunderclap headache, and patients tend to describe it as explosive, or "the worst headache I've ever had."

Mia Minen, a neurologist and chief of headache research at NYU Langone Health, says it's like "the snap of the finger: 10 out of 10 pain."

That stands out from a typical headache that tends to creep up over several minutes or an hour, linger a while and get worse as time wears on. Even though other non-life-threatening headache syndromes such as migraine or cluster headaches might still usher in the same pain intensity, those tend to build more gradually or may even be preceded by an aura (sensory symptoms such as tingling or spots in your vision).

Thunderclap headaches are a sign of something serious until proven otherwise, given their link to brain bleeds — things such as ruptured aneurysms or other hemorrhages, though they can be related to other masses or lesions in the brain as well.

Do not delay medical care with a headache such as this – it's an emergency, so call 911 or have someone help you to an emergency room. That is true for people of any age, with or without a history of headaches, and whether or not it's accompanied by other symptoms such as vomiting or passing out.

Nervous system symptoms

My patients and their families will tell you I often belabor the same list of stroke symptoms that we all need to be ready to recognize early. If a headache accompanies one of these symptoms, don't stop to think about the timing or severity, or really anything at all regarding that headache – just call 911.

The list of symptoms to watch for includes: abrupt weakness overall or in an arm or a leg, new numbness, changes in speech, confusion or trouble understanding and expressing thoughts clearly, facial droop or sudden trouble walking. It's also critical you get immediate care for changes in consciousness such as difficulty waking up, being in a stupor, passing out or having a seizure.

Changes from motion

This next group of symptoms deserve same-day attention by your primary care provider. If you don't have one, or can't get a prompt appointment, go to an urgent care clinic.

One of the most important questions they will ask you about the headache is what's making the pain better or worse. Take notice if the ache sharpens intensely with day-to-day movements — things such as bending over, rolling over in bed or even bearing down when you're having a bowel movement.