What evidence is there for or against acupuncture being an effective treatment?
Acupuncture is based on the belief that diseases are caused by blockages of your qi, which flows in so-called meridians in your body. By sticking needles into those meridians you can manipulate the flow of qi and eliminate the blockages.
The concepts of qi and meridians are unscientific, they date back to a time where there was no scientific method and knowledge of human anatomy was practically nonexistant. There is no evidence that qi and meridians exist at all.
Sticking needles into your body still could have some effect, although it will have nothing to do with your qi. There are tons of clinical studies about acupuncture, but few conclusive results. A major problem is that controlling for the placebo effect is complicated, as people usually notice whether you stick needles into them or not. Some of the newer and better clinical trials used fake acupuncture needles that do not penetrate the skin. Another often used method is to stick the needles outside of the "proper" acupuncture points as a control.
Acupuncture is believed to be helpful in a large variety of conditions, I'll take pain relief as an example as it is somewhat plausible that sticking needles into your skin could have an effect on the perception of pain.
A review from 2009 in BJ concludes:
A small analgesic effect of acupuncture was found, which seems to lack clinical relevance and cannot be clearly distinguished from bias. Whether needling at acupuncture points, or at any site, reduces pain independently of the psychological impact of the treatment ritual is unclear.
My conclusion is that acupuncture is just a particularly effective placebo.
It seems to be highly variable and condition-specific, although this latest review of reviews has some interesting results (mind you that's in paediatric population):
Efficacy and safety of acupuncture in children: An overview of systematic reviews. (highlighting is mine)
We included 24 systematic reviews, comprising 142 RCTs with 12787 participants. Only 25% (6/24) reviews were considered to be high quality (10.00 ± 0.63). High-quality systematic reviews and Cochrane systematic reviews tends to yield neutral or negative results (P=0.052, 0.009 respectively). The efficacy of acupuncture for five diseases (Cerebral Palsy (CP), nocturnal enuresis, tic disorders, amblyopia and pain reduction) is promising.
It was unclear for Hypoxic Ischemic Encephalopathy (HIE), Attention Deficit Hyperactivity Disorder (ADHD), mumps, Autism Spectrum Disorder (ASD), asthma, nausea/vomiting and myopia. Acupuncture is not effective for epilepsy. Only six reviews reported adverse events (AEs) and no fatal side effects were reported.
Linde, Klaus, et al. "Acupuncture for tension-type headache." Cochrane Database Syst Rev 1.1 (2009)., based on eleven trials with 2317 participants (median 62, range 10 to 1265) that met the inclusion criteria:
In the previous version of this review, evidence in support of acupuncture for tension-type headache was considered insufficient. Now, with six additional trials, the authors conclude that acupuncture could be a valuable non-pharmacological tool in patients with frequent episodic or chronic tension-type headaches.
You can find many more meta-reviews on http://onlinelibrary.wiley.com/cochranelibrary/search (search for "acupuncture"). E.g. here are more results (one paragraph per result):
There is low to moderate-level evidence that compared with no treatment and standard therapy, acupuncture improves pain and stiffness in people with fibromyalgia.
There is insufficient evidence to judge whether acupuncture is effective in treating cancer pain in adults.
Sham-controlled trials show statistically significant benefits; however, these benefits are small, do not meet our pre-defined thresholds for clinical relevance, and are probably due at least partially to placebo effects from incomplete blinding. Waiting list-controlled trials of acupuncture for peripheral joint osteoarthritis suggest statistically significant and clinically relevant benefits, much of which may be due to expectation or placebo effects.
The currently available evidence from a very heterogeneous group of randomised and quasi-randomised controlled trials evaluating the effects of acupuncture for the treatment of acute ankle sprains does not provide reliable support for either the effectiveness or safety of acupuncture treatments, alone or in combination with other non-surgical interventions; or in comparison with other non-surgical interventions. Future rigorous randomised clinical trials with larger sample sizes will be necessary to establish robust clinical evidence concerning the effectiveness and safety of acupuncture treatment for acute ankle sprains.
Due to a small number of clinical and methodologically diverse trials, little can be concluded from this review. There is little evidence to support or refute the use of acupuncture for shoulder pain although there may be short-term benefit with respect to pain and function. There is a need for further well designed clinical trials.
Acupuncture might be able to provide short-term relief from tennis elbow, but more research is needed. (http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD003527/abstract)
NIH's definition of acupuncture: *The general theory of acupuncture is based on the premise that there are patterns of energy flow (Qi) through the body that are essential for health. Disruptions of this flow are believed to be responsible for disease. Acupuncture may correct imbalances of flow at identifiable points close to the skin.
Is there evidence that acupuncture works? No, not within the scope of the NIH's definition, as there is no evidence that qi exists. But yes, within the scope of asking whether pain relief may be possible by mild deliberate trauma to the epidermis.