β-Chlornaltrexamine (β-CNA) is a non-selective irreversible antagonist of the μ-opioid receptor (MOR), the δ-opioid receptor (DOR), and the κ-opioid receptor (KOR), which forms a covalent bond to the binding sites of these receptors and has ultra-long-lasting opioid antagonist effects. Although it is predominantly antagonistic, β-CNA also shows some irreversible mixed agonist–antagonist activity at the MOR and KOR and some associated analgesic effects. Its alkylating group is a bis(chloroalkyl)amino-residue similar to that of the nitrogen mustards. The drug was first described by 1978. It should not be confused with its epimer and related drug α-chlornaltrexamine (α-CNA), which is likewise predominantly an irreversible antagonist of the opioid receptors but also shows some irreversible mixed agonist–antagonist activity.