Just wanted to throw something contradictory into the pot here, about contraindicated drugs and my own experiences:

The exact same drug that was provoking myasthenic weakness 8 months ago is the exact same drug that I was taking last spring when I went into quite a significant remission whilst in the process of actually taking it. (My progressive myasthenia felt like being on a 5 year roller coaster ride with pulls & releases, and at the very highest peak I got out, jumped and pulled my parachute landing 5 days later!!)

So what I learned in my own personal case here is that sometimes the drug dictates the myasthenia, but at other times the myasthenia has a life of its own and the power of a spontaneous remission from long term fatigable weakness can at times silence even the most offensive of drugs.