Vitamin D deficiency may intensify chronic pain

"Of the [chronic pain] patients tested, 26 percent had vitamin D inadequacy. Among these patients, the morphine dose was nearly twice that of the group with adequate vitamin D levels. In addition, the vitamin D inadequacy group used morphine for an average of 71.1 months versus 43.8 months. The vitamin D deficient group also reported lower levels of physical functioning and had a poorer view of their overall health. "

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Mind NOT over matter: the placebo is overrated

"...in a stunning 2001 article in the New England Journal of Medicine, two Danish physicians debunked the mighty placebo effect. Although most clinical trials include only active treatment and placebo groups, the researchers systematically collected all the studies in the medical literature (130 at the time) that also included a critical third group: patients who received neither active nor placebo treatment—just passive observation. The patients in placebo groups did report slightly less pain than the no-treatment groups; the analogy of a parent kissing a skinned knee comes to mind. But on almost every other objective measure of illness, the placebo-treated patients improved the same amount as the ones who got nothing at all. In other words, just believing you were getting treatment—the power of positive thinking—didn't really fix anything. It just made the patients hurt a little less."

-- from Slate, "Pill Popping: Debunking the Power of the Placebo Effect"

"No treatment" is the control group for the placebo-control group. In that context, the placebo effect actually seems very weak.