Headaches occur when pain-sensitive nerve endings called nociceptors react to headache triggers (such as stress, certain foods or odors, or use of medicines) and send messages through the trigeminal nerve to the thalamus, the brain’s “relay station” for pain sensation from all over the body.

The causes of many chronic daily headaches aren’t well-understood. True (primary) chronic daily headaches don’t have an identifiable underlying cause. Conditions that might cause nonprimary chronic daily headaches include: Inflammation or other problems with the blood vessels in and around the brain, including stroke.