<unk> (oral) is an analgesic opioid that is FDA approved for the treatment of drug detoxification - opioid abuse, opioid abuse, maintenance therapy, pain, chronic (moderate to severe), in patients requiring a continuous around-the-clock opioid analgesic for an extended period of time, pain (moderate to severe), not responsive to non-narcotic analgesics.. There is a Black Box Warning for this drug as shown here.  Common adverse reactions include cardiovascular: hypotension, endocrine metabolic: diaphoresis,  gastrointestinal: constipation, nausea, vomiting, neurologic: asthenia, dizziness, lightheadedness, sedated.
Methadone 
Levomethadone
Morphine 
Morpholine
Buprenorphine 
Codeine
Naltrexone 
Acetylmethadol