Hi,
Colicky abdominal pain and diarrhoea are a common side effects of medicine for Myasthenia gravis. Pyridostigmine is the medicine is the medicine which causes these side effects. The treatment is to either decrease the dose of the medicine or you can take propantheline (Pro-banthine) 30 minutes before each pyridostigmine dose to counteract the effects on the bowel.
Hope the information helps
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regards
Dr. Gupta