That's not an easy question to answer because there's not a lot of research on the long-term effects of the dietary supplement called methylsulfonylmethane (MSM). MSM is formed using dimethyl sulfoxide (DMSO), an organic form of sulfur that's also used as a dietary supplement.
Animals observed for 90 days on a daily dose of MSM five to seven times greater than what's 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. However, people taking a placebo also reported similar symptoms.
Since little is known about the long-term safety of MSM, it's no surprise that the efficacy also is unclear. Some studies have shown slight improvements in arthritis symptoms, but these studies were small and short term, so no definitive conclusions can be made.
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.