A study conducted between January 2001 and June 2006 examined the relationship between spinal manipulation and injuries. The results were published in the Journal of the Royal Society of Medicine in July, 2007 (Volume 100, pg. 330-338).
The study ultimately concluded that spinal manipulation can have negative effects and manipulation of the cervical spine can be especially dangerous. Side effects can be serious and include vertebral artery dissection and stroke. Although the study notes that the frequency of stroke is not known, the authors suggest rethinking the use of spinal manipulation as a form of routine treatment. For more information about this study please see http://jrsm.rsmjournals.com/cgi/content/full/100/7/330.