According to some anecdotal reports, MSM has been used in human research for many years in amounts above 2000 mg per day with no significant adverse effects.⁵ However, diarrhea, skin rash, headache, and fatigue may be experienced in less than 20% of people, according to other anecdotal reports. Detectable levels of MSM in the brain of a person taking MSM supplements have been reported,⁶ but the significance of this finding, if any, is unclear.