Migraine headaches are difficult to treat because the exact cause is unknown.
The most common theory is that right before the headache starts there is narrowing of blood vessels in the brain and blood flow is decreased. Then during the headache the blood vessels widen and blood flow in the brain is increased. Serotonin, a chemical in the brain, is thought to have a role in the migraine by causing the blood vessels to constrict. High levels of serotonin have been found in the brain just before the headache starts and lower levels have been found during the headache. This however is just a theory and may not be the cause of the migraine in every single case.