Acupuncture is a widely used alternative therapy
Traditional acupuncture is effective at preventing headaches, a scientific review finds - but so is a sham form.
The Cochrane Review reviewed 33 separate trials into acupuncture and its so-called "sham" counterpart.
The latter also involves the insertion of needles - but not into traditional "energy points".
The scientist leading the review said the results showed that putting needles into particular locations might not be that important.
Acupuncture is still regarded as a "complementary" therapy, but is increasingly being viewed as a potential mainstream treatment for certain conditions, such as chronic pain.
The finding by the Cochrane Collaboration is likely to lead to further calls for it to be made more widely available on the NHS.
The traditional explanation of its effects involves tapping into a network of "meridians" around the body to regulate the flow of an energy called "chi". Acupuncture points are located at various positions along these meridians.
However, many modern acupuncture specialists believe that the insertion of needles actually cause subtle changes in the nervous system and brain activity which can be beneficial - and place needles in other parts of the body rather than concentrating solely on traditional acupuncture points.
The Cochrane reviews involved a total of 6,736 patients, who were given acupuncture to prevent either mild to moderate "tension" headaches, or migraine attacks.
Following a course of at least eight weeks, acupuncture patients suffered fewer headaches than those given only painkillers.
'Not a sham'
Acupuncture was also superior to preventative drug treatments in migraine, the reviewers concluded.
However, acupuncture relying on non-traditional needle positions was just as good as the traditional variety in preventing tension headaches, and almost as good in the migraine patients.
Dr Klaus Linde, from the Centre for Complementary Medicine Research at the Technical University of Munich in Germany, said that much of the benefit for both might be due to a "placebo effect", in which the experience itself of being treated can produce results independently of the effects of the treatment.
He said: "Much of the clinical benefit of acupuncture might be due to non-specific needling effects and powerful placebo effects, meaning that the selection of specific needle points may be less important than many practicioners have traditionally argued."
Dr Mike Cummings, medical director of the British Medical Acupuncture Society, welcomed the research.
He said that the differences between so-called "true" and "fake" acupuncture remained controversial within the profession.
"I think that, quite literally, many practioners have missed the point in the past.
"We certainly don't call what we do 'sham' acupuncture, as we believe there is growing evidence for a mechanism behind what we do.
"However, we still don't fully understand what is happening when needles are inserted, although these reviews suggest that for certain conditions, it is effective."