everything I've read about MSM says that people who are allergic to sulfa medication have nothing to fear by taking MSM.
'MSM is a non-toxic substance, with a toxicity profile similar to that of water. The body uses the MSM that it needs and eliminates any excess within 12 hours.
MSM should not be confused with sulfites or sulfa drugs. Allergies to these substances do not affect your ability to take MSM. '