MSM (methyl-sulfonyl-methane), also known as dimethyl sulfone, is a naturally occurring sulfur compound, found in human diets and those of virtually all other vertebrates. In its purified form, it is an odorless, slightly bitter tasting, water soluble, white, crystalline powder containing 34% elemental sulfur. Its chemical formula is (CH3)2SO2.

Some atmospheric chemists have suggested that MSM and its related compounds, DMSO (dimethylsulfoxide), and DMS (dimethylsulfide) provide the source for 85% of sulfur compounds in all living organisms. These naturally occurring compounds begin in the ocean where phytoplankton convert inorganic sulfur present in the sea water to terniary dimethyl sulfonium salts. These salts are later broken down, by enzymatic action, into the volatile compound called dimethylsulfide (DMS), which escapes the ocean as a gas, rises into the upper atmosphere to be oxidized there in the presence of ozone and ultraviolet light to its chemical cousins, DMSO and MSM. Unlike DMS, both DMSO and MSM are water-soluble and are returned to the earth in rain. Plants rapidly absorb the two compounds, concentrating them up to a hundred-fold. Animals eat the plants and the distribution of these sulfur compounds is then complete. MSM has been found in the blood and adrenal glands of cows. Cows' milk contains between two and six parts per million MSM.

MSM occurs naturally in the human body as a result of the food we eat. It is a normal component of fresh fruits, vegetables, seafood and meat. It has also been found in tea, coffee and chocolate. It has been detected in normal human urine. The amount of MSM present in the circulatory system of an adult human male is about 0.2 parts per million. Normal adult humans excrete from four to eleven milligrams of MSM per day in their urine. In vertebrates, the concentration of MSM decreases with age. Some research suggests that there is a minimum concentration of MSM that must be maintained in the body to preserve normal function and structure.

Experiments using MSM that contains radiolabled sulfur (35S) have shown that after ingestion, MSM gives up its sulfur to the essential amino acids methionine, cysteine and other serum proteins, eventually finding its way into the collagen of skin, joints and blood vessels. It is also incorporated into the keratin of hair and nails.

Some people have found this substance to be useful for inflammation relief.