What is the truth behind Methadone, regarding my husband?

Is methadone a drug that can affect your personality in a negative way? My husband is on 100mg a day for almost 3 years. He was addicted to pain pills at one time and now this. He is irresponsible and selfish. I believe that this is causing a lot of strain on our marriage. He says that the methadone does nothing but helps him from going into withdrawals and there is no problem with being on it for the rest of his life. Is this true and am I over reacting?

: http://alcoholdrugstreatment.info/what-is-the-truth-behind-methadone-regarding-my-husband