Phenomorphan is the mu-opioid receptor agonist. Phenomorphan is a synthetic narcotic analgesic structurally related to racemorphan and racemethorphan. It has no accepted medicinal value in the United States.

Phenomorphan:
https://drugs.ncats.io/drug/26HXE4B73P

Morphinian-based Opioid With Fentanyl-esque potency:
https://www.reddit.com/r/researchchemicals/comments/4o99t2/morphinianbased_opioid_with_fentanylesque_potency/d4amw1v/

• A Synthetic opioid analgesic.
• It is not currently used in medicine.
• Has similar side-effects to other opiates, which include itching, nausea and respiratory depression.
• Phenomorphan is a highly potent drug around 10x more potent than levorphanol, which is itself 6-8x the potency of morphine.

Other Phenomorphan analogues are even more potent some being 60x and 45x stronger than levorphanol.