Did you know that research showed that compared to Medication and Acupuncture that spinal manipulation for chronic spinal pain syndrome was the only treatment that had a broad-based long-term benefit? 5 of 7 main outcome measures tested showed significant improvements compared with only 1 outcome measure in each of the acupuncture and the medication groups. The study was performed on 115 randomized patients during a 9-week treatment period. 
Another study showed significant improvement for sufferers of acute (short term-just happened) or chronic (long term) lumbar disc herniation. This study showed 90.5% of all the patients in the study were “improved” after 3 months with 88.0% “improved” still after 1 year. Acute patients responded better; however even 81.8% of chronic pain patients reported “improvement” after 3 months of care with 89.2% “improved” still after 1 year.
In a 2006 randomized controlled trial comparing spinal manipulative therapy to fake spinal manipulative therapy (the placebo) with disc herniation patients, showed that patients treated with real spinal manipulative therapy had greater pain relief and consumed fewer drugs compared with those receiving the fake spinal manipulative therapy.
The last study I will bore you with was published in September, 2001. Researchers did a comprehensive literature search using journal search engines like MEDLINE and EMBASE. Additionally they gathered all available data from the Cumulative Index of Nursing and Allied Health Literature, the Chiropractic Research Archives Collection, and the Manual, Alternative, and Natural Therapies Information System. They were looking for studies that were performed that showed the efficacy of spinal manipulative therapy in the treatment of headaches. You need to know that research is very hard. Mostly it is very expensive, costing a lot of time and money. To conduct a study you have to get some way to pay for it and some way to perform it. These researchers found 22 original studies that assessed the effect of spinal manipulative therapy in the treatment of headaches. These studies totaled 683 patients in the study groups. A total of 386 patients whose age ranged from 15 to 70 years received spinal manipulation. The others received medication, deep friction massage, palpation and rest, cold packs, or no treatment at all. Treatments ranged from 1 to 12 times over a period of 1 day to 8 weeks. Because this was all types of professional journals they used, the spinal manipulation or other therapy was performed by all types of practitioners and physicians like chiropractors, medical doctors, medical doctors with physical therapists, and osteopaths. Spinal manipulative therapy appeared to have a better effect than massage for cervicogenic (neck generated) headaches. It also appears that Spinal manipulative therapy had an effect comparable to commonly used first-line prophylactic (like anti-inflammatory) prescription medications for tension-type headaches and migraine headaches. 
As with all scientific studies, before drawing firm conclusions, further testing should be done in rigorously designed, executed, and analyzed trials with follow-up periods of sufficient length. I know that over 19 years in practice, my experience with joint manipulative therapy has done great things for all of my clients. If you know anyone who wants to fund a research project please send them my way.
P.S. I included the references for you to check should you want to read them.
 Long-term follow-up of a randomized clinical trial assessing the efficacy of medication, acupuncture, and spinal manipulation for chronic mechanical spinal pain syndromes. Reinhold Muller, PhD, et. al. ,Journal of Manipulative and physiological therapeutics, January 2005 Volume 28, Issue 1, page3-11.
 Outcomes of Acute and Chronic Patients With Magnetic Resonance Imaging–Confirmed Symptomatic Lumbar Disc Herniations Receiving High-Velocity, Low-Amplitude, Spinal Manipulative Therapy: A Prospective Observational Cohort Study With One-Year Follow-Up Serafin Leemann, DC, et. al. Journal of Manipulative and physiological therapeutics, March-April 2014, Volume 37, Issue 3, Pages 155–163
 Chiropractic manipulation in the treatment of acute back pain and sciatica with disc protrusion: a randomized double-blind clinical trial of active and simulated spinal manipulations. 7Santilli, V, Beghi, E, and Finucci, S., Spine Journal. 2006; 6: pages 131–137
 Efficacy of spinal manipulation for chronic headache: A systematic review, Gert Bronfort, DC, PhD, et al, Journal of Manipulative and physiological therapeutics, September 2001, Volume 24, Issue 7, Pages 457–466