Headaches have dozens of possible causes, but they fall into two broad categories: those that arise on their own from the way your brain and nerves process pain signals, and those triggered by something else going on in your body. In 2023, an estimated 2.9 billion people worldwide were affected by headache disorders, making them one of the most common health complaints on earth. Understanding what’s actually happening inside your head during a headache depends on which type you’re dealing with.
Primary vs. Secondary Headaches
Headache specialists divide all headaches into primary and secondary types. Primary headaches are conditions in their own right. They aren’t caused by another illness. Migraines, tension-type headaches, and cluster headaches all fall into this group. The pain comes from how your nervous system functions, not from damage or disease elsewhere in your body.
Secondary headaches are symptoms of something else. That “something else” can range from the completely ordinary (a hangover, skipping meals, a head cold) to the genuinely dangerous (a blood vessel problem in the brain, meningitis, or a tumor). Alcohol overuse, infections, head injuries, and fasting are among the most common secondary causes. The key feature of a secondary headache is that it improves or disappears once the underlying problem is treated.
Tension-Type Headaches
Tension-type headaches are the most familiar kind. They typically produce a dull, pressing sensation on both sides of the head, often described as a band tightening around your skull. The causes differ depending on whether you get them occasionally or nearly every day.
For occasional tension headaches, the problem is largely in the muscles around your head and neck. Pain-sensing nerve endings in these muscles, tendons, and the thin tissue covering them become activated. The frequency and intensity of the headache correlate directly with how tender your scalp and neck muscles are. Inflammation, reduced blood flow to the muscles, increased muscle activity, and even muscle wasting can all play a role. Many people also have “trigger points,” hyperirritable spots within a taut band of muscle that hurt when pressed. People with chronic tension headaches tend to have more of these trigger points in their head and neck muscles than people without headaches.
When tension headaches become chronic (occurring 15 or more days per month), the cause shifts. Constant pain signals from those muscles gradually change the way your central nervous system processes pain. Your brain’s pain-filtering systems stop working as well, and neurons in your brainstem become sensitized, meaning they overreact to stimuli that shouldn’t normally hurt. At that point, the headache is no longer just a muscle problem. It’s a nervous system problem.
What Causes Migraines
Migraines involve a different pain system entirely. The trigeminal nerve, the major nerve responsible for sensation in your face, plays a central role. During a migraine, branches of this nerve that wrap around blood vessels in the brain’s protective membranes become activated and release a signaling molecule called CGRP. This molecule dilates blood vessels, promotes inflammation around the brain, and amplifies pain signals being sent back to the brainstem.
CGRP is so central to migraine biology that an entire class of newer medications works by blocking it or its receptor. These were the first treatments designed specifically to target the trigeminal pain system rather than borrowing drugs developed for other conditions.
What triggers this cascade varies from person to person. Common triggers include hormonal shifts (especially around menstruation), stress, sleep disruption, weather changes, and certain foods. Specific dietary triggers include aged cheeses like cheddar, brie, and parmesan; cured and processed meats such as salami, hot dogs, and jerky; alcohol (especially red wine, beer, and sherry); MSG in its many forms; and the artificial sweetener aspartame. These foods contain compounds like tyramine and nitrites that can provoke the trigeminal system in susceptible people. Avocados, figs, citrus fruits, onions, and certain beans are also on the list of potential triggers.
Cluster Headaches
Cluster headaches are rarer but far more intense, often described as the worst pain a person can experience. Brain imaging studies have identified a specific area deep in the brain, the posterior hypothalamus, that activates during cluster attacks. This region appears to be the primary generator of the headache, and its involvement explains two hallmark features: the attacks follow a clock-like schedule (often striking at the same time each day) and they come in “clusters” lasting weeks or months before disappearing.
During an attack, the trigeminal nerve fires intensely on one side, and this triggers a reflex that floods the same side of the face with parasympathetic nerve activity. That’s why cluster headaches come with distinctive symptoms: a tearing, watery eye, a runny or congested nostril, and sometimes a drooping eyelid or constricted pupil, all on the same side as the pain. Blood pressure typically rises while heart rate drops during an episode.
Neck-Related Headaches
Problems in the cervical spine (the neck portion of your backbone) can refer pain into your head. These cervicogenic headaches originate from disorders of the bones, discs, or soft tissues of the neck. The pain is usually locked to one side and tends to radiate from the back of the head forward.
Key distinguishing features include reduced range of motion in the neck, pain that gets noticeably worse when you move your head or when someone presses on your neck muscles, and a clear timeline connecting the headache to a neck injury or condition. If the neck problem is successfully treated, the headache improves in parallel. Cervicogenic headaches can mimic migraines or tension headaches, which makes them easy to misdiagnose.
Dehydration and Other Physical Triggers
Dehydration causes headaches through a surprisingly physical mechanism. When your body loses enough fluid, your brain tissue actually shrinks slightly and pulls away from the skull. This tugging activates pain-sensing nerves in the membranes surrounding the brain. Rehydrating reverses the process, and the headache typically resolves once fluid levels are restored.
Other common physical triggers for secondary headaches include sinus infections (where inflammation and pressure in the sinus cavities refer pain to the forehead and cheeks), fever from any cause, eye strain, and changes in altitude or barometric pressure. Fasting and irregular meals can also provoke headaches, likely through drops in blood sugar and changes in stress hormones.
Medication Overuse Headaches
One of the more frustrating headache causes is the very medication you take to treat them. Medication overuse headaches develop when you use acute pain relievers too frequently over a period of more than three months. The threshold depends on the type of medication: for simple painkillers like ibuprofen or acetaminophen, using them on 15 or more days per month can trigger the cycle. For stronger or combination medications, the threshold is lower, at 10 or more days per month.
The result is a headache that occurs on 15 or more days per month and keeps getting worse the more medication you take. Nearly any class of pain reliever can cause this, from over-the-counter anti-inflammatories to prescription migraine medications and opioids. Breaking the cycle requires reducing or stopping the overused medication, which often means the headaches temporarily get worse before they get better.
Headaches That Need Urgent Attention
Most headaches are unpleasant but not dangerous. A few patterns, however, signal something potentially serious. A sudden-onset headache that reaches maximum intensity within seconds, sometimes called a “thunderclap headache,” can indicate a ruptured blood vessel in the brain and needs immediate evaluation.
Other warning signs include headaches accompanied by fever, night sweats, or unexplained weight loss; new neurological symptoms like weakness on one side, unusual numbness, or vision changes; a brand-new headache pattern starting after age 50; headaches that steadily worsen over weeks; headaches that change intensity when you shift position (standing to lying down); and headaches triggered by coughing or straining. New headaches during or shortly after pregnancy also warrant prompt evaluation, as they can point to vascular or hormonal complications. Any of these patterns suggests the headache may be a symptom of an underlying condition that needs its own diagnosis and treatment.