Methylsulfonylmethane, or MSM, is a synthesized dietary supplement ingredient. It delivers sulfur, an essential mineral nutrient. According to "The Miracle of MSM" by Drs. Jacob, Lawrence and Zucker, MSM relieves back pain, osteoarthritis, muscle pain, headaches, athletic injuries, carpal tunnel syndrome and other pain problems. While the authors state that MSM is without serious side effects, there are descriptions of some side effects in the scientific literature.
A 2004 osteoarthritis clinical trial conducted in India compared 1,500 mg/day of MSM to a placebo control for 12 weeks. The main adverse effect was diarrhea, which occurred in more than 5 percent of patients. The authors did not report on diarrhea frequency in the placebo group. None of the patients discontinued treatment due to an adverse reaction.
In the March 2006 issue of "Osteoarthritis and Cartilage," Dr. L.S. Kim reported on a placebo-controlled osteoarthritis clinical trail testing 6,000 mg/day for 12 weeks. At this high daily dose, there were three cases of gastrointestinal bloating in the treatment group and two in the control group. Neither group reported any cases of other gastrointestinal disorders such as stomach pain, gas, heartburn, vomiting or nausea.
According to "The Miracle of MSM," there are reports of headaches among people who start taking MSM at a high dose, say 5,000 mg/day. The authors recommend starting at a low dose and increasing over time.
Limitations of the two osteoarthritis clinical trials include that only relatively healthy adults ages 40 and older were tested, so there is no safety data for children, pregnant or lactating women, or people who are concurrently taking a broad range of prescription or over-the-counter drugs. MSM product labels usually have a cautionary statement about populations precluded from using the product, and a need to consult a physician before starting use.