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Varenicline   - trade name Chantix in the USA and Champix in Canada, Europe and other countries -  is a prescription medication used to treat smoking addiction.   It is not recommended for those under 18 years old or pregnant women. Women currently breastfeeding should also avoid this product since varenicline may pass into the breast milk leading to unknown effects on the child.

Varenicline is a partial agonist of the nicotinic acetylcholine receptor.

Theoretically, by acting as a partial agonist varenicline partially stimulates the ?4?2 receptor without producing a full effect like nicotine. Again, theoretically it is believed that “it does not greatly increase the downstream release of dopamine”. But this is the theory.

Nausea occurs commonly in people taking varenicline. Other less common side effects include headache, difficulty sleeping, and abnormal dreams, change in taste, vomiting, abdominal pain, flatulence, and constipation.

In November 2007, the FDA announced it had received post-marketing reports that patients using varenicline for smoking cessation had experienced several serious symptoms, including suicidal ideation and occasional suicidal behavior, erratic behavior, and drowsiness. On February 1, 2008 the FDA issued an alert, noting that "it appears increasingly likely that there is an association between Chantix and serious neuropsychiatric symptoms." As of July 1, 2009, the US Food and Drug Administration requires Chantix (varenicline) to carry a black box warning, the agency's strongest safety warning.  On June 16, 2011, the FDA issued a safety announcement that Chantix may be associated with a "a small, increased risk of certain cardiovascular adverse events in patients who have cardiovascular disease."

On July 4, 2011, four scientists published a review of double-blind studies in the Canadian Medical Association Journal. They found that varenicline has increased risk of serious adverse cardiovascular events compared with placebo.

The Institute for Safe Medication Practices (ISMP) conducted an analysis of post-marketing adverse effects reports received by the FDA. According to this analysis, in the fourth quarter of 2007 varenicline accounted for more reports of serious side effects than any other drug. Suicidal acts and ideation, psychosis, and hostility or aggression, including homicidal ideation, were the most prominent psychiatric side effects. Multiple reports suggested that varenicline may be related to the loss of glycemic control and new onset of diabetes, heart rhythm disturbances, skin reactions, vision disturbances, seizures, abnormal muscle spasms and other movement disorders.

On October 23, 2008, the Institute for Safe Medication Practices issued an analysis of prescription drug-related injuries reported to the FDA during the first quarter of 2008. According to the report, varenicline had more reported incidents than any other drug, with 1001 new cases of adverse effects and 50 more deaths reported.

On January 15, 2009, the Institute for Safe Medication Practices issued its analysis of prescription drug-related injuries reported to the FDA during the second quarter of 2008. During this period there were 910 newly-reported cases of serious injury attributed to varenicline, including 38 deaths. In addition to the above-reported psychiatric effects, the report noted increasing evidence linking varenicline to "potentially life-threatening allergic reactions." According to the report, varenicline had the second-highest number of new injury reports during this quarter.

On June 3, 2010, Health Canada also announced changes to the Canadian Product Monograph that include changes in mood or behaviour (such as depressed mood, agitation, aggression, hostility, thoughts of self-harm or harm towards others); serious allergic reactions (such as swelling of the face, lips, gums, tongue and throat that can cause trouble breathing) and skin reactions (such as rash, swelling, redness, and peeling of the skin); neuropsychiatric side-effects in patients taking varenicline with or without a history of psychiatric disorder; drinking alcohol increasing the risk of experiencing neuropsychiatric side effects; and side-effects such as sleepiness, dizziness, loss of consciousness, seizures, or difficulty concentrating. Health Canada advised those taking varenicline not engage in potentially hazardous activities, such as driving a car or operating dangerous machinery until they know how they may be affected by varenicline.