Headache is a pain in any part of the head. The pain in the headache can be sharp, throbbing, or dull and can occur on one or both sides of the head. Headaches can occur gradually or suddenly and can last for days or less than an hour. Headaches can be caused by a variety of factors, most often due to stress, but also by allergies, hormonal imbalances, poor digestion and absorption, low blood sugar, fatigue, and medications (including caffeine and alcohol). Most headaches are best treated by locating and removing the cause, a service that laboratory prevention tests can offer.