This is a randomized controlled trial performed at the University of Odense, Denmark by chiropractors and medical doctors.
From the abstract: Fifty-three [patients] suffering from frequent headaches who fulfilled the International Headache Society criteria for cervicogenic headache.were recruited from 450 headache sufferers from responded to the newspaper advertisements. .28 of the group received high-velocity, low-amplitude cervical manipulation twice a week for three wk. The remaining 25 received low-level laser in the upper cervical region and deep friction massage in the lower cervical/upper thoracic region, also twice a week for three weeks.
Results: The use of analgesics decreased by 36% in the manipulation group, but was unchanged in the soft-tissue group; this difference was statistically significant. The number of headache hours per day decreased by 69% in the manipulation group compared with 37% in the soft-tissue group; this was significant. Finally, the headache intensity per episode decreased by 36% in the manipulation group, compared with 17% in the soft-tissue group; this was significant. At a four-week follow-up, she remained pain free.
Nilsson N, Christensen HW, Hartvigsen J. J Manipulative Physiol Ther. 1997 (Jun); 20 (5): 326-330