Spinal manipulation is a regulated/controlled medical intervention and can only be performed by chiropractors and a limited number of physical medicine professionals. Prior to the adminstration of spinal manipulative therapy, absolute contraindications must be screened out.  These include inflammatory arthritides, fractures, dislocations, instabilities, bone weakening disorders, tumours, infections, acute trauma as well as various circulatory and neurological disorders. Although most contraindications apply only to spinal manipulation of the affected region, a few emergency conditions, such as visual field defects is an absolute contraindication spinal manipulative therapy.   In February 2008, the World Health Organization sponsored Bone and Joint Decade 2000-2010 Task Force on Neck Pain and its Associated Disorders, the largest and most comprehensive study on neck pain, diagnosis, prognosis and safety to date. This included a consensus of the top experts in the world whose findings were collated using a best-evidence synthesis, which addresses risk, prevention, diagnosis, prognosis and treatment risks and benefits. With respect to the association of VBA stroke and cervical manipulation the study concluded:

  Vertebrobasilar artery stroke is a rare event in the population.
  There is an association between vertebrobasilar artery stroke and chiropractic visits in those under 45 years of age.
There is also an association between vertebrobasilar artery stroke and use of primary care physician visits in all age groups.
no evidence of excess risk of VBA stroke associated chiropractic care.
 The increased risks of vertebrobasilar artery stroke associated with chiropractic and physician visits is likely explained by patients with vertebrobasilar dissection-related neck pain and headache consulting both chiropractors and primary care physicians before their VBA stroke."[http://www.fcer.org/html/News/bonejointdecade.htm  

A 2007 study which examined over 50 000 chiropractic spinal manipulations had no reports of serious adverse effects.   The most common minor side effect was  pain in the head, neck, arm or upper back; and stiffness of the neck, shoulder which occurred after, at worst, 16 in 1,000 (0.016%) treatments.  Furthermore, the authors concluded that the risk of serious adverse effects was, at worst, 6 per 100,000 (0.006%) spinal manipulations.   Despite the controversy and skepticism  regarding chiropractic spinal manipulation, the  World Health Organization states that  "[when] employed skilfully and appropriately, chiropractic care is safe and effective for the prevention and management of a number of health problems."