Side effects from Methadone are much like any other narcotic. If you get too hefty a dose, you'll get sleepy and lethargic. Otherwise, you should not get too many side effects. It should just take your pain away.
Constipation is one side effect that's common to almost all narcotics. There are some other less common side effects. Ask your pharmasist for a handout with them listed if you really want all of them.
Take care, guy
Dan