β-Funaltrexamine (β-FNA) is an irreversible (covalently bonding) opioid antagonist that was used to create the first crystal structure of the μ-opioid receptor (MOR).

[2] Chemically, it is a naltrexone derivative with a methyl-fumaramide group in the 6-position.

In addition to its MOR irreversible antagonism, β-FNA is a reversible agonist of the κ-opioid receptor (KOR) and produces KOR-mediated analgesic effects in animals.

[2][3][4] This has limited its usefulness and contributed to the development of methocinnamox as a more selective functionally irreversible antagonist of the MOR with no significant opioid agonistic actions.

[3] This drug article relating to the nervous system is a stub.