Get emergency medical help if you have any of these signs of an allergic reaction to methadone: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Even if you have used other narcotic medications, you may still have serious side effects from methadone.
Call your doctor at once if you have:
weak or shallow breathing;
severe constipation;
a light-headed feeling, like you might pass out; or
symptoms of a life-threatening heart rhythm disorder--a headache with chest pain and severe dizziness, and fast or pounding heartbeats.
Common methadone side effects may include:
dizziness, drowsiness;
nausea, vomiting; or
increased sweating.
This is not a complete list of side effects and others may occur. Tell your doctor about any unusual or bothersome side effect. You may report side effects to FDA at 1-800-FDA-1088.