Most of methadone's side effects are likely to be more pronounced at the beginning of treatment — during the two- to six-weeks of treatment. The most frequently reported side-effects include drowsiness and light-headedness, nausea and vomiting, excessive sweating, dry mouth, weakness, constipation and change in sex drive. Most patients build tolerance to these side effects as the treatment goes on and the dose is adjusted by the physician. The physician can also investigate and prescribe other medications to alleviate these side effects, if they tend to persist.
This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.