<unk> (transdermal) is an analgesic that is FDA approved for the treatment of moderate to severe chronic pain in patients requiring a continuous, around-the-clock opioid analgesic for an extended period of time. There is a Black Box Warning for this drug as shown here.  Common adverse reactions include pruritis, nausea, vomiting, constipation, xerostomia, headache, and dizziness.
Buprenorphine 
Methadone 
Naltrexone 
Tapentadol
Morphine 
Acetylmethadol
Tramadol
Levomethadone