Varenicline-d 4 hydrochloride is a deuterium labeled Varenicline (dihydrochloride). Varenicline (CP 526555) dihydrochloride is a potent partial agonist for α4β2 nicotinic acetylcholine receptor ( nAChR ) with an EC 50 value of 2.3 μM. Varenicline dihydrochloride is a full agonist for α3β4 and α7 nAChRs with EC 50 values of 55 μM and 18 μM, respectively. Varenicline dihydrochloride is a nicotinic ligand based on the structure of cytosine, and has the potential for smoking cessation treatment.