MSM is the abbreviation for methylsulfonylmethane, or organically bound sulfur. MSM contains more than 34% elemental sulfur and also contains two methyl groups. MSM is naturally found in rainwater, fresh fruits and vegetables (1 to 4 mg MSM per kg). MSM is relatively volatile and dissipates easily upon heating or prolonged storage. Breast milk is the richest natural source of MSM.