this is some information i found about it on drugs.com, hope it helps.
-Methadone is a opioid pain reliever, similar to morphine. An opioid is sometimes called a narcotic. It also reduces withdrawal symptoms in people addicted to heroin or other narcotic drugs without causing the "high" associated with the drug addiction.
-Taking methadone improperly will increase your risk of serious side effects or death. Even if you have used other narcotic medications, you may still have serious side effects from methadone. Follow all dosing instructions carefully.
-Methadone may impair your thinking or reactions. Avoid driving or operating machinery until you know how methadone will affect you.
these are some side effects of using it.
shallow breathing;hallucinations or confusion;chest pain, dizziness, fainting, fast or pounding heartbeat; ortrouble breathing, feeling light-headed, or fainting.
Less serious methadone side effects may include:
feeling anxious, nervous, or restless;
sleep problems (insomnia);
feeling weak or drowsy;
dry mouth, nausea, vomiting, diarrhea, constipation, loss of appetite; or
decreased sex drive, impotence, or difficulty having an orgasme side effects