Org 27569 is a drug which acts as a potent and selective negative allosteric modulator of the cannabinoid CB<sub>1</sub> receptor. Studies in vitro suggest that it binds to a regulatory site on the CB<sub>1</sub> receptor target, causing a conformational change that increases the binding affinity of CB<sub>1</sub> agonists such as CP 55,940, while decreasing the binding affinity of CB<sub>1</sub> antagonists or inverse agonists such as rimonabant. However while Org 27569 increases the ability of CB<sub>1</sub> agonists to bind to the receptor, it decreases their efficacy at stimulating second messenger signalling once bound, and so in practice behaves as an insurmountable antagonist of CB<sub>1</sub> receptor function.