Varenicline is an orally administered small molecule with partial agonist activity at the alpha4beta2 nicotinic acetylcholine receptor. Varenicline was approved by both the US FDA and the European Medicines Agency of the EU in 2006 as an aid to smoking cessation. Subsequently, varenicline has been approved in over 80 other countries. Varenicline is almost entirely absorbed following oral administration, and absorption is unaffected by food, smoking or the time of day. Varenicline undergoes only minimal metabolism and approximately 90% of the drug is excreted in the urine unchanged. Varenicline has a mean elimination half-life after repeated administration of approximately 24 hours in smokers. The area under the plasma concentration-time curve is increased in patients with moderate or severe renal failure. No clinically relevant varenicline-drug interactions have been identified. In two identical, randomized, double-blind, phase III clinical trials in healthy, motivated-to-quit, mainly Caucasian smokers aged 18-75 years in the US, 12 weeks of treatment with varenicline 1 mg twice daily was associated with significantly higher abstinence rates over weeks 9-12 than sustained-release bupropion 150 mg twice daily or placebo. In a separate phase III trial, an additional 12 weeks of treatment in smokers achieving abstinence in the first 12 weeks was associated with greater abstinence through to week 52 than placebo treatment. Varenicline treatment was also associated with significantly higher rates of abstinence than placebo treatment in randomized, double-blind, clinical trials in smokers in China, Japan, Korea, Singapore, Taiwan and Thailand. In a randomized, open-label, multi-national, phase III trial, varenicline treatment was associated with a significantly higher rate of abstinence than transdermal nicotine-replacement therapy. In these trials, varenicline treatment was associated with lower urge to smoke and satisfaction from smoking in relapsers than placebo or active comparators. In the two US phase III trials, 12 weeks of treatment with varenicline 1 mg twice daily had an acceptable safety and tolerability profile. Nausea and abnormal dreams were the most common adverse events that occurred in more varenicline than placebo recipients. The incidence and prevalence of nausea were greatest in weeks 1 and 2 of treatment, and declined thereafter. The prevalence of early adverse effects can be reduced by individual dose titration. Adverse events associated with varenicline therapy have been reported in post-marketing surveillance, including neuropsychiatric events such as depressed mood, agitation, changes in behaviour, suicidal ideation and suicide. Currently, it is unclear whether the association of varenicline therapy with these adverse events is causal, coincidental or related to smoking cessation. Given the greater efficacy of varenicline compared with other pharmacotherapies, and the high risk of morbidity and mortality associated with continued smoking, varenicline is a valuable pharmacological aid to smoking cessation.