Hey, Linda. I'll give you the scientific reason.
The body has a lot of "checks and balances" systems in it. Like when you're dehydrated, the antidiuretic hormone kicks in and helps you absorb what water you do have in your body. If you're overhydrated, it backs off and lets your body get rid of the fluid.
In MG, antibodies attack the neuromuscular junction (NMJ), where the nerves and muscles come together. You get either damaged or destroyed muscle receptors, where the acetylcholine (ah-seat-ill-co-lean) is supposed to go in to make the muscles strong.
Acetylcholine (ACh) is the "muscle gas" that makes everyone's muscles strong. When ACh is done with its job of making our muscles strong, there's an enzyme called Acetylcholinesterase (AChE) that comes along and "mops up" whatever ACh is leftover.
Mestinon is called a cholinesterase inhibitor. It inhibits that AChE enzyme, so that we can use what acetylcholine is in our NMJ for a longer period of time.
Too much of that muscle gas and our NMJ gets overloaded and we get weaker. That's what can cause a "Cholinergic Crisis."
Too little muscle gas or other treatments and we can get a "Myasthenic Crisis."
For some people, they can take a steady dose of Mestinon only a few times a day. I do better on 90 mg. every three hours, round the clock. I don't even need to look at a clock to know what time I need it, though I write down all of my doses/times anyway. I can feel it run out. Some people take an extra dose when they run errands. I usually take an extra 10 mg. or so when I do that and then after I get home.
It takes awhile to get used to the drug and to know what works for you. Just keep having a conversation with your neuro about it. I'm sure he would really like to know about the dramatic response you are getting. Mine sure did!
Does that answer your question? Sue already gave the answer as to what variables are involved in how effective it is or not.
Give yourself time. Revel in feeling better! I don't know what I'd do without Mestinon.
Annie