After thousands of years of practice, we know that acupuncture works, but the West is still trying to understand why. Acupuncture derives from the concept in traditional Chinese medicine that disease results from a disruption in the flow of chi—the body's circulating life energy—and imbalances in the forces of yin and yang. Chi is said to flow along pathways in the human body known as meridians. According to Eastern thought, there are as many as 20 meridians and more than 2,000 acupuncture points found along them. Applying tiny needles—or, sometimes, pressure or heat—to those points is believed to deliver therapeutic effects for patients.

The success of acupuncture is evident in its popularity in the U.S. and its growing acceptance among medical professionals. A 2002 National Center for Health Statistics survey estimated that 8.2 million adults had tried acupuncture. Acupuncture needles are regulated by the Food and Drug Administration, and the therapy is sometimes approved by insurance.

But just how it works is a mystery Western researchers are still attempting to understand. In a consensus statement issued by the National Institutes of Health in 1997, scientists acknowledged that some of the tenets of acupuncture—like the flow of chi and the network of meridians—is "difficult to reconcile with contemporary biomedical information," but argued that there is "clear evidence" that acupuncture works for treatment of postoperative and chemotherapy-induced nausea and vomiting. They also found evidence of pain relief from conditions such as postoperative dental pain, menstrual cramps, tennis elbow, and fibromyalgia. In other words, we don't know how it works, but it seems to. Here are a few ways going under the needles may alleviate pain.

Releasing natural painkillers

One theory is that acupuncture stimulates the release of the body's own painkillers, such as endorphins.

Putting up a roadblock

The "gate control theory" posits that acupuncture may activate peripheral nerves to shut the "gate" on pain signals traveling through the spinal cord. The idea of interrupting pain signals is also the basis for another alternative therapy—transcutaneous electrical nerve stimulation (TENS).

"The gate control theory of pain is one of the predominant theories of pain right now," says John Lefebvre, PhD, a professor of psychology at Wofford College in Spartanburg, S.C. "If you can shut the gates down, you can eliminate the pain. Acupuncture seems to close the gates most of the way."

Helping the brain take control

For other scientists, acupuncture's effects may be, at least in part, connected to patients' beliefs. Some research has indicated that patients' expectations of the benefits plays a role in their experience. "This is at some level getting at the placebo response," says Sean Mackey, MD, chief of the pain management division at Stanford University School of Medicine, in Palo Alto, Calif. "I don't want to suggest that we view acupuncture as a kind of voodoo magic. There's clearly something going on, we're still trying to understand it. I believe there is a large central—i.e., brain-related—component to this, and for some people it can be very effective."

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