With the possible exception of head colds, headaches are the most common human ailment. Usually headaches are merely passing annoyances that go away with aspirin or after a nap, but as many as 45 million people suffer from chronic and/or severe headaches that seriously interfere with their lives.

Like colds, headaches are not completely understood by scientists. There appear to be various types of headaches, but any hard and fast classification is open to debate, in part, because the types often overlap - both in symptoms and in response to medication. Moreover, triggering factors and modes of relief vary from person to person. Still, the great majority of primary headaches (those not due to underlying diseases) falls into three categories, according to the International Headache Society: tension, migraine and cluster.

The vast majority of headaches are benign and self-limiting. Common causes are tension, migraine, eye strain, dehydration, low blood sugar, and sinusitis. Much rarer are headaches due to life-threatening conditions such as meningitis, encephalitis, cerebral aneurysms, extremely high blood pressure, and brain tumors. When the headache occurs in conjunction with a head injury the cause is usually quite evident. A large percentage of headaches among women are caused by ever-fluctuating estrogen during menstrual years. This can occur prior to, or even during midcycle menstruation.

Treatment of an uncomplicated headache is usually symptomatic with over-the-counter painkillers such as aspirin, paracetamol (acetaminophen), or ibuprofen, although some specific forms of headaches (e.g., migraines) may demand other, more suitable treatment. It may be possible to relate the occurrence of a headache to other particular triggers (such as stress or particular foods), which can then be avoided.