I don't know about that one in particular, Petch. Are we talking about MSM in powder form, or in liquid? I would think a powder would probably be okay.
My main concern with the original question was the mixing of the two carrier substances, especially with respect to the minoxidil. I believe that introducing any form of moisture into a minoxidil bottle will hasten the breakdown of the minoxidil carrier substance and render it less effective. So, in effect, it is the cream vehicle that is carrying the retin a that I would be worried about, not the retin a per se... nor the MSM per se in your case.
If the MSM you are taking is in liquid form, then I would not mix the MSM liquid with the minoxidil and store it that way. I keep them separate, and only mix them only for that day's batch.