Patient description: The patient is a 16-year-old girl recently diagnosed with myasthenia gravis, class IIa. She complains of diplopia and weakness affecting in her upper extremities. She had a positive anti-AChR antibody test, and her single fiber electromyography (SFEMG) was positive. She is on acetylcholinesterase inhibitor treatment combined with immunosuppressants. But she still has some symptoms. She does not smoke or use illicit drugs. She is not sexually active, and her menses are regular. Her physical exam and lab studies are not remarkable for any other abnormalities. BP: 110/75 Hgb: 11 g/dl WBC: 8000 /mm3 Plt: 300000 /ml Creatinine: 0.5 mg/dl BUN: 10 mg/dl Beta hcg: negative for pregnancy

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  Myasthenia Gravis diagnosed with myasthenia gravis and who are currently taking pyridostigmine and experienced pyridostigmine-related GI side effects within the past 7 days. 2. GSRS rating of at least Moderate discomfort on questions 5, 11, and 12. 3. Subjects must be willing and able to complete a GI symptom diary within a consistent timeframe on a daily basis. 4. Subjects must be able to tolerate a pyridostigmine dose of 30mg TID. 5. Must be clinically stable in judgement of treating neurologists for past 3 months. 6. Must have AchR antibody positive MG. 7. Subjects must be able to swallow liquids. 8. Subjects must be in good health as determined by their medical history, physical examination, vital signs, and laboratory tests. A subject with a medical abnormality may be included only if the investigator or designee considers that the abnormality will not introduce significant additional risk to the subject's health or interfere with study objectives. 9. Subjects must have signed an informed consent form indicating that they understand the purpose of and procedures required for the study and are willing to participate in the study and comply with the study procedures and restrictions  Key The for of a subject from enrollment in the study are as follows: 1. Any acute or chronic diseases which are associated with GI distress (such as nausea, vomiting, or diarrhea), which could interfere with the subjects' safety during the trial, expose them to undue risk, or interfere with the study objectives. 2. History or presence of hepatic, or renal disease or other condition known to interfere with the absorption, distribution, metabolism, or excretion of drugs. 3. History of substance abuse, known drug addiction, or positive test for drugs of abuse or alcohol. 4. Patients currently using marijuana for any reason (medical or recreational). 5. Known hypersensitivity to pyridostigmine, or to ondansetron or similar 5-HT3 serotonin receptor antagonists. 6. ECG changes including QT interval prolongation and congenital long QT syndrome. Electrolyte abnormalities (e.g., hypokalemia or hypomagnesemia), congestive heart failure, bradyarrhythmia's or other medicinal products that lead to QT prolongation. 7. Treatment with drugs affecting peripheral cholinergic transmission within 1 month of study entry (with the exception of pyridostigmine). 8. Subjects unlikely to co-operate during the study, and/or be questionably compliant in the opinion of the investigator. 9. Patients currently being treated with narcotics. 10. Patients being treated with aminoglycoside antibiotics, which are contraindicated in myasthenia gravis. 11. Patients unable to be contacted in case of an emergency. 12. Intake of an investigational drug within 30 days of study entry. 13. Pregnancy and women of childbearing potential not willing to follow the birth control requirements as described in the informed consent or breastfeeding. 14. History or presence of obstructive pulmonary disease or urinary obstruction (contraindication for pyridostigmine). 15. This use of selective serotonin reuptake inhibitors (SSRI's)