<unk> (mucous membrane) is an opioid analgesic that is FDA approved for the treatment of chronic, severe pain in patients requiring long-term daily around-the-clock opioid analgesic.   Common adverse reactions include application site erythema, application site irritation, application site rash, pruritus, constipation, nausea, vomiting, xerostomia, dizziness, headache, and somnolence.
Buprenorphine 
Methadone 
Naltrexone 
Tapentadol
Morphine 
Acetylmethadol
Tramadol
Levomethadone