There is no Research into possible MSM Toxicity in Horses. The only research I can find in the literature has been done in rats.
The Equine Industry continues to use MSM in Equine Supplements but is it safe. Stamina Plus has used MSM in horses and is currently conducting in a research project to evaluate any potential toxicity.
This research is ongoing but data at this time shows there may be a limit on how long MSM can be feed and at what level.
As this project is completed in the next few months Stamina Plus will provide more information.