A cluster headache is one-sided head pain that may involve tearing of the eyes and a stuffy nose. Attacks occur regularly for 1 week to 1 year, separated by long pain-free periods that last at least 1 month, possibly longer.

See also:

• Headache
• Migraine headache
• Tension headache

Cluster headaches are a fairly common form of chronic, repeated headaches. They are more common in men than women. The headaches can occur at any age but are most common in adolescence and middle age. The tend to run in families.

Scientists do not know exactly what causes cluster headaches, but they appear to be related to the body's sudden release of histamine or serotonin.

The following may trigger cluster attacks:

• Alcohol and cigarette smoking
• High altitudes (trekking, air travel)
• Bright light (including sunlight)
• Exertion
• Heat (hot weather, hot baths)
• Foods high in nitrites (such as bacon and preserved meats)
• Certain medications (including nitroglycerin and various blood pressure medications)
• Cocaine