In my book, Alternative Medicine: The options, the claims, the evidence, how to choose wisely, I have a chapter discussing acupuncture in which we conclude:

Now, according to a report in HealthDay News, “Needle insertion stimulates production of chemical known to reduce discomfort, scientists say.” This may now explain how acupuncture works. Here are the details:
The needle pricks involved in acupuncture may help relieve pain by triggering a natural painkilling chemical called adenosine, a new study has found.
The researchers also believe they can enhance acupuncture’s effectiveness by coupling the process with a well-known cancer drug — deoxycoformycin — that maintains adenosine levels longer than usual.
“Acupuncture has been a mainstay of medical treatment in certain parts of the world for 4,000 years, but because it has not been understood completely, many people have remained skeptical,” lead author Dr. Maiken Nedergaard, co-director of the Center for Translational Neuromedicine at the University of Rochester Medical Center, said in a news release.
“In this work, we provide information about one physical mechanism through which acupuncture reduces pain in the body.”
Nedergaard and her team report their findings online in the journal Nature Neuroscience. They are also scheduled to present the results this week at the Purines 2010 scientific meeting in Barcelona.
Working exclusively with mice, Nedergaard and her colleagues administered half-hour acupuncture treatments to a group with paw discomfort.
The investigators found adenosine levels in tissue near the needle insertion points was 24 times greater after treatment, and those mice with normal adenosine function experienced a two-thirds drop in paw pain.
By contrast, mice that were genetically engineered to have no adenosine function gained no benefit from the treatment.
The team also found that if they activated adenosine in the same tissue areas without applying acupuncture, the animals’ discomfort was similarly reduced, strongly suggesting that adenosine is the magic behind the method.
Adenosine, better known for regulating sleep, inhibits nerve signals and inflammation, the authors explained.
In their experiments with deoxycoformycin, which is known to impede adenosine removal from the body, the researchers said the drug almost tripled the amount of adenosine in the targeted muscles and more than tripled the amount of time that the mice experienced pain relief.
The study was funded by the New York State Spinal Cord Injury Program and the U.S. National Institutes of Health. And, you can read more about acupuncture in my book, Alternative Medicine: The options, the claims, the evidence, how to choose wisely.