@fBoyle-- It's fine for this drug to be used this way, but some doctors are even prescribing it for pain that is less than severe because it's effective and affordable. That's a terrible idea because it's just setting up people for addiction.
Methadone is believed to be as addictive as narcotics like heroin. It stays in the body for a very long time but it's pain-relieving effects don't last as long. So patients are tempted to take more and more. If a doctor prescribes this drug, there has to be a very tight control on the dosage and it should not be used longer than necessary. As patients express less pain, the dose should be tapered and eventually stopped altogether. The longer one uses it, the more difficult withdrawal will be.