Just as the name implies, a headache is a pain in all or part of the head that may also be felt in the face and neck. A headache can be a symptom of disease, in which case it is known as a secondary headache. Infectious diseases, such as the common cold, influenza, and pneumonia, often produce secondary headaches, as does an allergic reaction or a sinus infection. Secondary headaches are also a symptom of the eye disease glaucoma, high blood pressure, stroke, head injury, a brain tumor, and a teeth-grinding condition known as temporomandibular joint (TMJ) syndrome.

Ninety percent of all headaches, however, are unrelated to any other medical condition. These headaches are called primary headaches. Primary headaches may be episodic, which means they occur occasionally, or they may be chronic. Chronic headaches strike at least four times a...