Someone who has experienced a serious injury -- in football, for example-- will usually report that initially the pain was bearable but with time the pain became much worse. One reason pain seems less intense immediately after injury is that the brain produces endorphins.

Endorphins are chemicals produced by the brain and secreted in response to injury or severe physical or psychological stress. The pain-reducing properties of endorphins are similar to those of morphine, a powerful painkilling drug.

The brain produces endorphins is situations that evoke great fear, anxiety, stress, or bodily injury, as well as after intense aerobic activity. For example, subjects showed increased levels of endorphins after being stressed by receiving painful electric shocks or by holding their hands in ice water. Patients showed increased levels of endorphins after their teeth nerves were touched or their bandages were removed from badly burned areas of the body. These studies indicate that the brain produces endorphins to reduce pain during periods of intense physical stress. Endorphins and other painkillers (heroin, morphine, codeine) act mostly to stop receptors from signaling severe, persistent pain but do not stop receptors from signaling quick, sharp pain as from a pinprick.

Researchers have identified the generic code responsible for the development of the endorphin-morphine receptor in the brain. This receptor, which is activated by morphine, heroin, or endorphin, is the key to both reducing pain and causing addiction. Researchers hope that, as they better understand how this receptor works, they can develop drugs that reduce pain but are not addicting.

Not only does the brain make endorphins, but so too do the body's adrenal glands, which make an endorphin-like chemical that reduces pain. Researchers removed pain-reducing adrenal gland cells from individuals who were brain-dead and transplanted these cells into the spinal cords of patients suffering from chronic and severe cancer pain. Four of five patients who received transplanted cells reported dramatic decreases in pain. Researchers were very encouraged and suggested that transplants of adrenal gland cells that make an endorphin-like chemical represent a promising approach to treating chronic pain.

Endorphins may also be involved in explaining how acupuncture reduces pain.