Studies suggest that MSM has a protective effect against oxidation, and potentially inflammation, in athletic horses. MSM is a naturally-occurring sulfur compound which contains anti-oxidant properties. 
 
While it’s not yet clear how MSM works in the equine body, it is a great source of dietary sulfur, which plays an important role in maintaining the health of collagen, cartilage, hooves, hair, and joint fluid.
MSM is said to have the following effects. MSM is also said to be great at treating skin conditions such as itch. Other effects may include the reduction of scar tissue, muscle repair, an anti-inflammatory effect, supporting joint health and boosting the immune system. 
 
MSM is incredibly safe to feed with no known negative side effects reported.  Note that it is likely to take several weeks for effects to appear.
 
Dosage for a 500kg horse- 5 to 20 grams day in the feed.