All drugs cause some side effects. However, some cause minor and some cause major side effects. Patients using Mestinon mostly complained of having: Nausea and vomiting, Diarrhea, Stomach cramps or pains, Increased saliva, Chest congestions, Low blood pressure, Skin rash, Muscle cramps and twitching Contact your doctor or seek medical help if you experience any of the following symptoms: Shortness of breath, Wheezing or difficulty breathing, Swelling of the face, lips, tongue or other parts of the body, Rash, itching or hives on the skin, Slowed heart rate, Shortness of breath, Severe rash, Irritation, Swollen face