I've been on it for almost a year. It worked for my MG: not as much as a nice high dosage of prednisone, but it worked better than all the others - which all didn't do a thing (azathioprin, cellcept, etc).
I just stopped cold turkey, had to because it already caused irreverisble damage to my liver, and is not like prednisone where you cannot stop just like that.

I did not have any problems, though after a few weeks my MG got a bit more room to play around. But just to compare: 2,5 mg less of the prednisone is much more MG problems than stopping the MTX 25 mg I took (injected) weekly. For me anyway! I do not know or can predict what will happen to you - just so you know.

Are you on any other drugs? Because if the MTX is the only one you use, and it works good for you, fat change stuff will get worse if you stop. Again; no diagnosis or whatever.

Why do you have to stop?