MSM, which is about one-third sulfur, acts as a dietary source of sulfur. Sulfur is involved in a wide variety of metabolic pathways and plays an important structural role in amino acid and protein metabolism. Sulfur is required for proper synthesis and maintenance of connective tissues such as skin, hair, nails, tendons and cartilage. Many supplements claim MSM to be a dietary treatment for osteoarthritis based on the presence of sulfur in connective tissues such as collagen (collagen comprises nearly three quarters of the solid portion of cartilage).
Despite the wide range of anecdotal reports of MSM effectiveness, there is little compelling scientific evidence supporting such claims – particularly for osteoarthritis. Several small animal studies have suggested that MSM may play a role in resistance to stress and stimulation of immune system responses. Doses in the range of 1-5mg/kg/d (approximately 70-350mg for an average-sized man) over a period of 2-4 weeks appear to stimulate synthesis of immunoglobulins (in mice and chickens). In horses, larger doses (2.5-10 grams per day) have been associated with improvements in hoof quality.
The best news about MSM is that it can be considered very safe (though not very effective) when used as a dietary supplement. In rats and dogs, toxic effects are reported only for extremely high doses – which would correspond to well over 200 grams per day for an average-sized man (about 8 ounces of the stuff!).
As a dietary sulfur source (its only valid benefit) MSM would appear to be an overpriced supplement option. There are a number of other less expensive, yet equally effective dietary sources of sulfur, including eggs, meat and fish – as well as sulfur containing amino acids such as methionine and cystine/cysteine. Large doses of methionine, however, should also be accompanied by supplemental levels of key strong vitamins such as folic acid, B6 and B12 – which are known to reduce homocysteine levels (homocysteine is a metabolite of methionine and high levels have been associated with an increased risk for cardiovascular disease).
Typical dosage recommendations range from 2-5 grams per day as a beginning or "loading" dose to about 50-200mg per day for maintenance. Due to the lack of strong scientific efficacy, however, MSM is not recommended as a particularly effective dietary supplement for joint health.