Mestinon is used for the treatment of myasthenia gravis. In patients who suffer from myasthenia gravis the muscles quickly become tired and weak and in severe cases the muscles may become paralysed. Myasthenia gravis is caused by excessive activity in the body of a protein called cholinesterase. Mestinon belongs to a group of medicines known as cholinesterase inhibitors. Cholinesterase inhibitors stop the excessive activity of cholinesterase and in this way help muscles to work properly. Mestinon is also used to treat some rare forms of constipation (paralytic ileus) and inability to pass urine after an operation.