<prompt> Varenicline is used inPulmonary hemosiderosisSleep apneaAnti-trypsin deficiencyNicotine dependency  <response>: Varenicline is a prescription medication used to treat smoking addiction. This medication is the first approved nicotinic receptor paial agonist. Specifically, varenicline is a paial agonist of the alpha4beta2 subtype of the nicotinic acetylcholine receptor Ref Davidson 23rd edition pg 578 