If you have lactose intolerance, that could be the problem, at least in part. The mestinon pills use lactose as the filler (the stuff they mix the actual medicine with to be able to pack the pills into shape.)

I found that using a lactase pill (over the counter, like Lact-Aid or Dairy-Ease) with each dose of mestinon helped tremendously. Worked it down to using just 1/2, then 1/4, of the lactase tablet, which seemed to be enough for me.

Taking it with some food also seems to help, as suggested above. something to temper the effect of the pill on empty stomach and guts.

Why the manufacturers use lactose in pills, knowing so many people have trouble with it, is beyond me.