Methadone is a long-acting opioid medication used under medical supervision for specific conditions. It may be prescribed for severe, persistent pain when other treatments are inadequate, and it is also used as part of medically supervised treatment for opioid use disorder. 

Because methadone can cause addiction, misuse, respiratory depression, and overdose, it should only be taken exactly as prescribed by a qualified healthcare professional. Never purchase methadone from unverified websites or use someone else’s prescription. Combining methadone with alcohol, benzodiazepines, or other medicines that slow the nervous system can dangerously suppress breathing. 

Common side effects may include nausea, vomiting, constipation, sweating, dizziness, drowsiness, and lightheadedness. More serious warning signs include slow or difficult breathing, fainting, chest pain, confusion, or a fast or irregular heartbeat. Seek emergency medical help if these occur.