Common mental and emotional side effects of methadone hydrochloride include anxiety, nervousness, restlessness, decreased sex drive and sleep problems, according to RxList. Common physical side effects include drowsiness, dry mouth, nausea, diarrhea and loss of appetite.
If a patient experiences shallow breathing, hallucinations, confusion, chest pain or trouble breathing while taking this medication, he requires immediate medical attention, warns RxList. In addition, he needs to seek emergency medical help if he displays signs of an allergic reaction, such as swelling of the face, lips or throat, difficulty breathing or hives. Methadone interacts with other medications, such as narcotic pain relievers, sedatives, muscle relaxers, heart medications and blood pressure medications.