A headache is pain or discomfort originating from pain-sensitive structures in and around your head. Your brain tissue itself cannot feel pain. Instead, the sensation comes from a network of nerves, blood vessels, muscles, and membranes that surround and protect the brain. Roughly 40% of the global population, about 3.1 billion people, experiences a headache disorder in any given year.
Why Your Brain Doesn’t Actually Hurt
The brain has no pain receptors. What you feel during a headache is signaling from the structures around it: the skin, muscles, and blood vessels in your head and neck, the membranes that wrap around the brain, large arteries near the base of the skull, major drainage channels inside the skull, and cranial nerves. Even sinus linings and dental structures can generate head pain. When any of these tissues are stretched, compressed, inflamed, or irritated, nearby nerves fire pain signals that you experience as a headache.
At a chemical level, a key player is a signaling molecule called CGRP. When the nerve system connecting the brain to the face and head becomes activated, it releases CGRP, which dilates blood vessels and triggers an inflammatory response that produces pain. At the same time, levels of serotonin, a chemical that normally keeps blood vessels constricted and pain in check, tend to drop. With less serotonin to counterbalance it, the vessel-widening effect goes unopposed, and pain intensifies.
Primary vs. Secondary Headaches
Every headache falls into one of two broad categories. A primary headache is the condition itself, not caused by another disease. Tension-type headaches, migraines, and cluster headaches are all primary. A secondary headache is a symptom of something else: an infection, a head injury, a blood vessel problem, medication withdrawal, or even severely high blood pressure. If a new headache appears alongside or shortly after another medical condition known to cause head pain, it’s classified as secondary regardless of what the pain feels like. You can even have both at once. If a long-standing migraine pattern suddenly worsens because of a new underlying cause, both diagnoses apply.
Tension-Type Headaches
Tension-type headache is the most common primary headache and the second most prevalent health condition on earth, affecting over 2 billion people, roughly a quarter of humanity. The pain is typically mild to moderate, felt on both sides of the head, and often described as a pressing or tightening band. It doesn’t throb the way a migraine does, and it usually won’t stop you from walking or climbing stairs, though chronic cases can cause significant suffering and impair daily functioning.
Migraines
A migraine is more than a bad headache. It’s a neurological event that can unfold in distinct phases over hours or even days. Untreated attacks last anywhere from 4 to 72 hours.
The first phase, called the prodrome, can begin a day or two before the pain. You might notice unusual fatigue, yawning, food cravings, or a dip in mood. About one in three people with migraines then experience an aura: temporary neurological symptoms that typically appear as visual disturbances like shimmering zigzag lines or blind spots. Some people get tingling or numbness spreading across the face or arm, and a smaller number have difficulty with speech.
The headache phase itself tends to be one-sided, pulsating, and moderate to severe. Routine physical activity like walking makes it worse. Nausea, vomiting, and extreme sensitivity to light and sound are common. After the pain subsides, the postdrome phase can leave you feeling drained or foggy for another day.
For some people, migraines become chronic, meaning headache on 15 or more days per month for more than three months, with at least 8 of those days meeting migraine criteria.
Cluster Headaches
Cluster headaches are rarer than migraines or tension-type headaches but far more intense. People consistently describe the pain as a 10 out of 10, centered around or above one eye. The pain is explosive and unrelenting, not a dull ache. Unlike migraines, which often make people want to lie still in a dark room, cluster headaches cause extreme restlessness. People pace, rock back and forth, or in some cases bang their heads against a surface to distract from the pain.
Individual attacks last between 15 minutes and 3 hours and can strike up to eight times a day, though twice daily (often at night) is most common. They come with distinctive autonomic symptoms on the same side as the pain: tearing or red eye in 90% of cases, nasal congestion or a runny nose in 84%, and eyelid swelling or facial swelling in about 59%. These symptoms help distinguish cluster headaches from other types.
Common Secondary Headache Causes
Secondary headaches can stem from a wide range of conditions. Head or neck trauma, including whiplash and concussion, is one of the most frequent triggers. Infections that cause fever, from the flu to meningitis, commonly produce headaches. Blood vessel disorders such as stroke, arterial tears, or inflammation of the temporal arteries (particularly in people over 65) can present with head pain as a primary symptom.
Other causes include severely elevated blood pressure (typically when systolic pressure reaches 180 or higher), increased pressure of spinal fluid inside the skull, sinus and dental problems, and brain tumors, though tumors are a relatively uncommon cause. Substance use and withdrawal, including alcohol and caffeine, also belong in this category.
When a Headache Is an Emergency
Most headaches are not dangerous, but certain patterns signal something serious. Neurologists use a set of red flags to identify headaches that need urgent evaluation:
- Thunderclap onset: a severe headache that reaches maximum intensity within one minute. This can be the only initial symptom of a brain bleed (subarachnoid hemorrhage).
- Neurological symptoms: weakness, confusion, vision loss, difficulty speaking, or decreased consciousness alongside a headache. Headache occurs in about one in four strokes.
- Fever with headache: raises concern for a brain or systemic infection, or an inflammatory condition.
- New headache after age 65: older adults with a new headache pattern are more likely to have a serious secondary cause than younger people.
- Cancer history: a new headache in someone with a known cancer is highly suspicious for spread to the brain.
- Pattern change: a sudden shift in your usual headache pattern, or an entirely new type of headache, can be the only sign of a serious underlying problem.
Medication Overuse Headache
One of the most common and least recognized headache triggers is the very medication used to treat them. When you take pain relievers too frequently, your nervous system adapts, and the headaches come back more often and more stubbornly. This creates a cycle where you take more medication, which worsens the problem.
The threshold depends on the type of medication. For basic pain relievers like acetaminophen or ibuprofen, using them on 15 or more days per month for more than three months crosses the line. For stronger or combination medications, the threshold is lower: 10 or more days per month for over three months. Breaking the cycle typically requires reducing or stopping the overused medication, which can temporarily make headaches worse before they improve.