Along with its needed effects, varenicline may cause some unwanted or undesirable effects. When stopping to smoke, regardless of whether medication is used to help, the following symptoms may occur: depression, short-temperedness, frustration, anger, nervousness, lack of patience, difficulty with concentration, increased appetite and weight gain. Possible unwanted effects of taking varenicline are mild to moderate and usually occur in the first weeks of treatment. They include:

• nausea
• vomiting
• trouble sleeping
• headache
• abnormal dreams
• constipation
• gas

Rarely:

• Allergic reaction such as redness, itching or swelling of the skin, hives, burning, stinging, or any other skin problems, swelling of the neck area, or any difficulty with breathing, not present before using varenicline.

Some people have had changes in behavior, hostility, agitation, aggression, depressed mood, and suicidal thoughts or actions while taking this medication. These psychiatric symptoms have occurred in people with previous mental health issues, as well as in those with no previous history. Some people had these symptoms when they started the medication, others developed them later. Drinking alcohol may increase the risk of developing these symptoms.
If you, your family, or caregiver notice agitation, hostility, depression or changes in behavior or thinking that are not typical for you, or you develop any of the following symptoms, stop taking this medication and call your healthcare provider right away:

• thoughts about suicide or dying, or attempts to commit suicide
• new or worse depression, anxiety or panic attacks
• feeling very agitated or restless
• acting aggressive, being angry, or violent
• acting on dangerous impulses
• an extreme increase in activity and talking (mania)
• abnormal thoughts or sensations
• seeing or hearing things that are not there (hallucinations)
• feeling people are against you (paranoia)
• feeling confused
• other unusual changes in behavior

Varenicline does not contain nicotine, but it has been shown to affect the nicotine receptor thought to be most related to smoking addiction. Varenicline can affect this receptor in two opposite ways: it acts like a weaker version of nicotine (which would reduce withdrawal symptoms), and also blocks nicotine from getting to the receptor (which would reduce the pleasure of smoking), but the exact way the drug works in people is not known.

• The effects of changes in the body resulting from stopping smoking, with or without treatment with varenicline may alter the way other drugs work. Tell your doctor if you are taking insulin, asthma medications (eg. theophylline), blood thinners (eg. warfarin). The dose of these medications may need to be adjusted once you are smoke free. Tell the doctor if you are taking any other prescription or non prescription medications. A lower dose may need to be used if you have kidney problems.
• Varenicline may affect your mental alertness or physical coordination. Do not engage in potentially hazardous tasks, such as driving a car or operating dangerous machines until you are sure of the affect of varenicline on mental alertness or physical coordination.
• The safety and benefits of taking varenicline with other products for stopping smoking (eg. patches, gum, inhaler) have not been studied. Therefore, it is not recommended to take varencline with these products. Using these products with varenicline is not likely to increase your chance of stopping smoking and may increase side effects.
• Use is not recommended in the following situations::
  • allergy to varenicline or any component of the preparation
  • when also taking nicotine replacement therapy, such as patches, gum or inhaler. The combination may result in increased side effects.
• Caution is recommended in the following situations:
  • when taking insulin
  • when taking some asthma medications
  • when taking some blood thinners
  • kidney problems
  • history of seizures
  • heart disease

Use in pregnancy and breastfeeding: The use of this medication is not recommended if pregnant or breastfeeding.