Headaches may be caused by a number of conditions, such as disorders of the neck, eyes, brain, jaw or teeth. Headaches with an underlying medical condition are classified as secondary headaches because they are related to the condition. An example of this would be a headache due to neck injury or sinus infection.
Other headaches are classified as primary because the headache itself is the main medical problem, although underlying non-medical causes, such as muscle tension or foods, may be identified. Other contributing factors may include medications, dehydration or changing levels of hormones. These factors that influence headaches are sometimes called headache triggers.