– A randomized controlled trial reported by medical doctors and doctors of chiropractic in Denmark found manipulation to have “a significant positive effect” on intensity and duration of cervicogenic headaches compared to “soft-tissue” therapy. (Nillson, Christensen, and Hartrigsen 1997).

– Boline et al. (1995) in JMPT conducted a study comparing manipulation to pain medication (amitriptyline) in the treatment of tension headaches. The authors found that pain medication had short-term effectiveness – although with side effects – while “ four weeks after the end of intervention, the spinal manipulation group showed a 32% reduction in headache intensity, 42% (reduction) in headache frequency, 30 % (reduction) in over-the-counter medication usage, and a 16% improvement in functional health status. The amitriptyline therapy group showed no improvement or slight worsening” (150).

– “Cervical spine manipulation was associated with significant improvement in headache outcomes in trials involving patients with neck pain and/or neck dysfunction and headache.” – Duke Evidence Report, McCrory, Penzlen, Hasselblad, Gray (2001).

References:
Nillson, Niels, Hendrik Wulff Christensen, and Jan Hartvigsen. 1997. “The Effect of
Spinal Manipulation in the Treatment of Cervicogenic Headache.” Journal Of Manipulative and Physiological Therapeutics20, no. 5: 326-330.
Boline, Patrick D., Kassem Kassak, Gert Bronfort, Graig Nelson, and Alfred V. Anderson. 1995. “ Spinal Manipulation vs. Amitriptycline for the Treatment of Chronic Tension-Type Headaches: A Randomized Clinical Trial.” Journal of Manipulative and Physiological Therapeutics 18, no. 3: 148-154.