Migraine headache is a common neurological condition that occurs in about 20% of the population and in 50% of women. It is not clear how a migraine works, but it is believed that the basic cause is an abnormality of serotonin, which is a chemical used by the brain cells. During a migraine, changes in serotonin levels cause the blood vessels in the brain to constrict. This decreases oxygen supply in the brain. In rare cases, a stroke is possible.