This thread is nearly 3 years old and I'm sure you won't get a response from lawsn, at this point.
I have been on Mestinon for only 2 months and am gaining weight, too. Thought it might be because I cant exercise at the previous intensity, hence muscle waste. My GP says that he isnt aware that the Mestinon can cause weight gain - perhaps it is more about the lethargy than the medication? I have decided to do some maintenance exercises using resistance bands, pilates ball, wrist weights and will see if it makes a difference. Never-the-less, Mestinon is amazing. I feel like it has given me my life back.