A US trial funded by the National Institutes of Health suggesting that more intensive chiropractic care produces superior results for patients with chronic headaches

80 patients were randomly assigned to 1 of 4 groups.

One of the groups received chiropractic manipulation once weekly for 8 weeks (8 visits), or twice weekly for 8 weeks (16 visits). In both these groups patients received light massage and a hot pack prior to manipulation.
Primary interest was in pain reduction at 12 weeks, which was approximately one month after treatment ended.
At 12 weeks there was “a clinically important and statistically significant” advantage in pain reduction for the patients receiving chiropractic manipulation. These patients also had significantly better results on various secondary measures at 12 weeks – reduction number of headaches during the past 4 weeks; reduction in disability and “unpleasantness” related to headache; and reduction in pain and disability related to neck pain.

Further, those in the chiropractic manipulation group receiving 8 treatments had a 9.4-point advantage in pain reduction over those in the controle groups, whereas those receiving 16 visits for chiropractic manipulation had a 17.2-point advantage.
Therefore in answer to the main question posed by the trial, whether there is a dose response for the more intensive course of care, these results suggest there is – with 16 visits producing better results than 8 visits for these patients with chronic CGH (Cervicogenic Headache).

References:
Reported at WFC Congress/ECU Convention 2007