Q. What exactly happens when I eat something cold and get an ice-cream headache? Is it harmful in any way?

A. Ice-cream headache, also known as "brain freeze" or cold-stimulus headache, is a headache some people get when they consume a cold food or beverage quickly. The pain is usually in the forehead or both temples, and it usually lasts less than five minutes.

The cause is debated, but most experts believe it starts when a cold substance touches the roof of the mouth or the back of the throat and causes small blood vessels in those areas to constrict and then rapidly dilate. Pain receptors near the blood vessels sense the discomfort and send the message along tiny nerve fibers to a larger nerve (the trigeminal nerve), which forwards it to the brain. The trigeminal nerve also carries pain signals from the face. The brain reads the cold-stimulus sensations as coming from the head rather than the mouth -- a phenomenon called referred pain.

Cold-stimulus pain is common, occurring in 30 percent to 40 percent of people who don't usually have headaches. The symptoms are harmless and not a sign of any underlying disease, although many experts believe they're more common in migraine sufferers.

Because ice-cream headaches are so short-lived, they're hard to study, and there's no consensus on how to stop them. Most people have their own methods; the most common is to curl the tongue and press the underside against the roof of the mouth. The best way to prevent the headache is to eat very cold foods slowly. -- Celeste Robb-Nicholson, M.D., Editor in Chief, Harvard Women's Health Watch

(c) 2009 Harvard Health Letters