I've been taking Mestinon for years, and have never heard of it being toxic. Pehaps he was refering to the fact that one CAN overdose on Mestinon. the only real drawback I've noticed to mestinon is that over time your body can become adjusted to it, so that you need to up the dosage to get the same effect. I've found that coming off the Mestinon for a time, perhaps several days, lets you sort of re-set everything. Of course, with my relatively mild MG, I'm able to tolerate a couple of days without Mesty, where someone else probably couldn't.