There's tremendous overlap between the symptoms of fibromyalgia and some of the symptoms of vitamin D deficiency. In some people, low vitamin D causes a body-wide pain and a hypersensitivity to touch, along with fatigue and depression - very similar to fibromyalgia. There are differences, though. For example, fibromyalgia classically involves trigger points.
Now a small study shows that vitamin D helps fibromyalgia patients. This was a randomized, blinded, controlled study, which is pretty high quality design. The numbers were low - 30 patients - but the study reached statistical significance despite that, which argues that the effect of vitamin D was strong.
So anyway, they took 30 patients with fibromyalgia, randomized them, and used supplementation to increase vitamin D levels from the 20's to around 50 ng/ml. After a few weeks, the group getting vitamin D had lower pain and higher levels of physical function.
Not bad for a stupid vitamin. Of course, as I have noted previously, vitamin D appears to lower death rates, so we know something's going on here. I never said this stuff was a miracle cure for everybody - but it appears to be doing some good for at least some people.