That 116 million estimation of chronic pain in the U.S. comes from a lengthy Institute of Medicine (IOM) report released last June, 2011: Relieving Pain in America: A Blueprint for Transforming Prevention, Care, Education, and Research [discussed in UPDATE here]. Apparently, many healthcare providers and opinion leaders find that number a bit far-fetched. Where did it come from and can it be trusted as being valid?
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http://updates.pain-topics.org/2011/11/many-readers-have-questioned-latest.html