Abnormal cannabidiol is a synthetic regioisomer of cannabidiol that fails to elicit either central cannabinoid (CB) or peripheral cannabinoid (CB) receptors responsiveness and is without psychotropic activity. It induces endothelium-dependent vasodilation via a CB/CB/nitric oxide-independent mechanism. O-1821 is a cannabidiol analog with close structural similarity to O-1918 which is a selective antagonist of abnormal cannabidiol at the non-central cannabinoid (CB)/peripheral cannabinoid (CB) receptors endothelial receptor. O-1918 does not bind to CB or CB receptors at concentrations up to 30 µM and inhibits the vasorelaxant effects of abnormal cannabidiol in vitro and in whole animals. The biological activity of O-1821 has not been reported.