Objective: To ensure that 16 weeks of methylsulfonylmethane (MSM) does not cause adverse effects in patients with the musculoskeletal disorders of osteoarthritis and back pain. Design: We carried out a subgroup analysis on data from a randomized, double-blind, placebo-controlled trial, "The use of Methylsulfonylmethane (MSM) in the treatment of low back pain," to determine the safety of taking 6 g daily of MSM (OptiMSM (R), Bergstrom Nutrition). We monitored metabolic parameters to determine whether MSM altered hematologic, liver or kidney function. We also monitored physiologic parameters of blood pressure and weight. Setting: Family Medicine Residency, Mike O'Callaghan Military Medical Center. Main outcome measures: Metabolic parameters as measured by hematologic function - white blood cells (WBC), platelets, hemoglobin (Hb), glucose; liver function as measured by - total bilirubin, alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Physiologic parameters as measured by weight, diastolic (DBP) and systolic blood pressure (SBP); kidney function as measured by creatinine. Results: Analysis of outcome measures showed no significant difference between MSM and placebo (p < 0.05) safety values. Conclusion: MSM has no effects on WBC, platelets, Hb, total bilirubin, AST, ALT, creatinine weight, DBP, or SBP in this study.