Healing hands: Chiropractors insist their treatments are safe
Chiropractic treatment on the neck carries a small risk of tearing an artery - and causing a stroke, say researchers.
It may also make pre-existing artery tears worse, they say.
The doctors who carried out the study say that, in the absence of any evidence backing the effectiveness of chiropractic for neck pain, the risks of the treatment might outweigh the benefits.
However, a UK chiropractic body insists that the therapy is safe and effective - and that the study is flawed.
Thousands of Britons consult chiropractors for a variety of complaints ranging from back pain to headaches.
The latest research, published in the journal Neurology, looked at dozens of cases of "cervical arterial dissection" - a tearing of an artery near the spine - followed by a stroke arriving at the University of California at San Francisco Medical Center and Stanford Medical Center over a five year period.
They found that these patients were six times more likely to have had spinal manipulation in the 30 days prior to their stroke than people who had had a different kind of strokes.
Dr Wade Smith, who led the study, said: "These observations suggest that spinal manipulative therapy can directly produce dissection.
"It is highly improbable that a young patient will have a stroke and have had spinal manipulative therapy within seconds purely by chance, given the relatively low frequency of both events."
It is irresponsible to suggest that chiropractic has caused these effects based on a study of this small size
Professor Alan Breen, General Chiropractic Council
However, he points out: "The number of chiropractic manipulations performed in the US each year is in the millions, and the incidence of stroke from all causes is only 10 per 100,000 - so we're not talking about large numbers of victims.
"But rare incidences do happen and physicians and patients should be aware of spinal manipulation therapy as a rare but potentially causal factor in stroke."
The researchers wrote in their study: "In the absence of randomised controlled trial evidence demonstrating the effectiveness of cervical manipulation, the best current evidence suggests that the small risk of dissection and stroke outweighs the benefit of this treatment for patients with acute neck pain."
However, Professor Alan Breen, a member of the General Chiropractic Council, was critical of the study.
He said: "It is irresponsible to suggest that chiropractic has caused these effects based on a study of this small size.
"What you have to remember is that someone who is in the process of having a dissection often experiences neck pain - precisely the symptom that might influence them to seek help from a chiropractor.
"Many of the people in this study who have gone on to suffer a stroke may have been experiencing an arterial dissection before they decided to consult a chiropractor."