Magnesium is a muscle relaxant because it displaces calcium, which contracts muscles. That is not what the purpose of magnesium is with fibromyalgia. Fibromyalgia is in part a reduction of ATP (adenosine triphosphate) in the muscle cells. Magnesium helps to increase ATP levels. It is generally taken as magnesium malate because the malic acid (malate) is also a precursor for ATP formation. Magnesium malate is sold in health food stores. Recommended dose is 300mg 2 to 3 times daily.
Fibromyalgia has also been linked to low serotonin levels, so supplementation with 5-HTP, a serotonin precursor is also recommended.