MSM is short for Methylsulfonylmethane, which is a natural compound found in some foods. As a supplement, it's often used in combination with glucosamine and chondroitin for arthritis and other joint issues, but it's also supposed to be helpful for PMR, according to my naturopath and some online research I've done. I have noticed some difference since I've been on it, although it took a couple of months to take effect. It's not a cure, of course, but I do find it helpful.