Few studies have examined the long-term effects of the dietary supplement called methylsulfonylmethane (MSM), which is formed by oxygenation of dimethyl sulfoxide (DMSO), an organic form of sulfur also used as a dietary supplement. Animals observed for 90 days on a daily dose of MSM five to seven times greater than that typically used in people had no serious problems. Stomach upset, diarrhea and headache have been reported in human trials of MSM lasting up to 12 weeks.
Given the lack of data about the long-term safety of the supplement, it's no surprise that its efficacy is also unproven. According to a review of studies involving people treated for spinal arthritis with glucosamine, chondroitin or MSM, no MSM studies published between 1984 and 2009 were conducted carefully enough for useful statistical analysis. An earlier review of studies testing MSM for arthritis in any joint found two small trials suggesting that the supplement may be helpful for knee arthritis.
Although there is great interest in using MSM to treat a variety of conditions, including arthritis, more research is needed to determine its benefits and risks.