Difelikefalin TFA is a κ-opioid receptor (KOR) agonist. It activates KOR in HEK293 cells expressing the human receptor (EC50 = 0.16 nM in a transactivation assay) and inhibits forskolin-induced cAMP production in R1.G1 mouse thyoma cells (EC50 = 0.048 nM). Difelikefalin TFA is selective for KOR over the μ-opioid receptor (MOR; EC50 = >1 µM in a transactivation assay). It reduces acetic acid-induced writhing, as well as scratching behavior induced by the KOR antagonist GNTI, in mice (ED50s = 0.07 and 0.05 mg/kg, respectively).