I have a herniated disk that causes SEVERE muscle spasms in my back. My doctor prescribed Flexeril. I asked about taking a muscle relaxer along with Mestinon. My doc said that they do not interfere with each other as each works via completely different pathways. So far it has relieved my spasms quite well and I notice no undue weakness. I don't understand how this works but two physicians have told me the same thing.

I would be very careful to specifically ask your doctor about this before taking a muscle relaxer. Just because one is deemed "safe" by a doctor doesn't mean they all are if you have MG.