Headache is pain in any region of the head. Headaches may occur on one or both sides of the head, be isolated to a certain location, radiate across the head from one point, or have a viselike quality.

A headache may appear as a sharp pain, a throbbing sensation or a dull ache. Headaches can develop gradually or suddenly, and may last from less than an hour to several days.

Your headache symptoms can help your doctor determine its cause and the appropriate treatment. Most headaches aren't the result of a serious illness, but some may result from a life-threatening condition requiring emergency care.

Headaches are generally classified by cause:

See a doctor if you experience headaches that:

• Occur more often than usual
• Are more severe than usual
• Worsen or don't improve with appropriate use of over-the-counter drugs
• Keep you from working, sleeping or participating in normal activities
• Cause you distress, and you would like to find treatment options that enable you to control them better

A secondary headache is a symptom of a disease that can activate the pain-sensitive nerves of the head. Any number of conditions — varying greatly in severity — may cause secondary headaches.

Possible causes of secondary headaches include:

• Acute sinusitis
• Arterial tears (carotid or vertebral dissections)
• Blood clot (venous thrombosis) within the brain — separate from stroke
• Brain aneurysm (a bulge in an artery in your brain)
• Brain AVM (arteriovenous malformation) — an abnormal formation of brain blood vessels
• Brain tumor
• Carbon monoxide poisoning
• Chiari malformation (structural problem at the base of your skull)
• Concussion
• Coronavirus disease 2019 (COVID-19)
• Dehydration
• Dental problems
• Ear infection (middle ear)
• Encephalitis (brain inflammation)
• Giant cell arteritis (inflammation of the lining of the arteries)
• Glaucoma (acute angle closure glaucoma)
• Hangovers
• High blood pressure (hypertension)
• Influenza (flu) and other febrile (fever) illnesses
• Intracranial hematoma (blood vessel ruptures with bleeding in or around the brain)
• Medications to treat other disorders
• Meningitis (inflammation of the membranes and fluid surrounding your brain and spinal cord)
• Monosodium glutamate (MSG)
• Overuse of pain medication
• Panic attacks and panic disorder
• Persistent post-concussive symptoms (Post-concussion syndrome)
• Pressure from tight headgear, such as a helmet or goggles
• Pseudotumor cerebri (idiopathic intracranial hypertension) (increased pressure inside the skull), also known as idiopathic intracranial hypertension
• Stroke
• Toxoplasmosis
• Trigeminal neuralgia (as well as other neuralgias, all involving irritation of certain nerves connecting the face and brain)

A headache can be a symptom of a serious condition, such as a stroke, meningitis or encephalitis.

Go to a hospital emergency room or call 911 or your local emergency number if you're experiencing the worst headache of your life, a sudden, severe headache or a headache accompanied by:

• Confusion or trouble understanding speech
• Fainting
• High fever, greater than 102 F to 104 F (39 C to 40 C)
• Numbness, weakness or paralysis on one side of your body
• Stiff neck
• Trouble seeing
• Trouble speaking
• Trouble walking
• Nausea or vomiting (if not clearly related to the flu or a hangover)

A primary headache is caused by overactivity of or problems with pain-sensitive structures in your head. A primary headache isn't a symptom of an underlying disease.

Chemical activity in your brain, the nerves or blood vessels surrounding your skull, or the muscles of your head and neck (or some combination of these factors) can play a role in primary headaches. Some people may also carry genes that make them more likely to develop such headaches.

Some types of secondary headaches include:

• Ice cream headaches (commonly called brain freeze)
• Medication overuse headaches (caused by overuse of pain medication)
• Sinus headaches (caused by inflammation and congestion in sinus cavities)
• Spinal headaches (caused by low pressure or volume of cerebrospinal fluid, possibly the result of spontaneous cerebrospinal fluid leak, spinal tap or spinal anesthesia)
• Thunderclap headaches (a group of disorders that involves sudden, severe headaches with multiple causes)