I have been asked this question countless times. What I would often like to do is give a sarcastic response. 'Well I don't know but I figure since I spent all that time and money getting my degree and license, I just keep at it.' Instead I say something like, "In my clinical experience, yes, it works." People usually try acupuncture after they have tried everything else. And they almost always leave pleasantly surprised.
Acupuncture has been around for thousands of years. Teachers have passed on their skill, their lineage, and their clinical wisdom to their students. If it didn't work, I imagine somewhere in all those years, clinicians would have realized it didn't work and they would have tried something else. People everywhere exclaim yes, acupuncture works, despite biomedicine's scientific double-blind tests' inability to grant acupuncture the clinical respect it deserves.
How does it work? It moves stagnation out of excess areas of your body, and directs energy to places that are deficient, and it stimulates your body to heal itself. (fMRI study of how acupuncture stimulates the limbic system of the brain). Therefore, it can treat just about anything. People often come to acupuncture for one major complaint and find that a handful of other complaints improve.
Western medical language doesn't deal with Qi and meridians. In order to get a picture of how acupuncture works, one has to be open to learning a whole new view of the body. Our Western view of the body is as a machine with disparate parts. There are systems that work independently and there are layers of muscle, tissue, bone, fluids, and blood.
Acupuncture's view of the body takes all of that into account but also sees the subtle levels. It sees the body less as a machine and more like a network of rivers, channels, estuaries of flowing and stagnating energy. Each part is connected to another and to the whole. The individual is also connected and influenced by nature and the world - microcosm and macrocosm. Bringing a person to health means balancing all of these influences as they exist uniquely in each body.
A couple weeks ago, I was treating an MD and he remarked that he would never have given acupuncture a try if his wife hadn't forced him. I replied with a laugh that it was funny because I avoid western docs unless I feel like I'm dying. Luckily, he laughed too. I certainly respect Western Medicine, but taking a pill to mask a pain while causing a host of other problems doesn't appeal to me or many other folks. I would rather get to the root of the illness, if I can.
A good acupuncturist will not only needle you, they will give you a diagnosis that you can understand and advise you on how to best nurture yourself. If we incorporate acupuncture and the wisdom of Eastern medicine into our lives, we can certainly keep the doctor away. :)