Champix is a highly successful quit-smoking medication, clinically proven to contribute to successful smoking cessation in up to a quarter of patients who tried it. Due to its reliability, extremely positive reviews and the fact that it’s often recommended by doctors all over the world, Champix is a very sought-after medicine, preferred by those who found nicotine replacement therapies and other alternatives ineffective. As with all modern medicaments, Champix has gone through many rigorous clinical trials which provided crucial information not only about its effectiveness but also about possible side effects and interactions with other drugs and substances. But, in order to gain an informed and detailed insight into these questions, one should also be familiar with the way varenicline, the active ingredient of Champix, works and how it interacts with the body.

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Varenicline mechanism of action

Chemically, varenicline is categorized as a high-affinity nicotinic receptor partial agonist. What this means is that it binds to the α4β2 nicotinic acetylcholine receptor subtype (nACH) that leads to the release of dopamine in the nucleus accumbens when activated. At the same time, varenicline competes with nicotine for receptor occupancy. With this unique mechanism of action, Champix both diminishes withdrawal symptoms and prevents nicotine molecules from binding to those receptors, resulting in decreased pleasurable effects from consumption of tobacco products.

So, it is important to note that Champix works by directly interfering with the neural connections that are at the core of the physiological aspects of nicotine addiction. This fact is extremely important for acquiring a proper understanding of its possible side effects which are usually rooted in the same or close areas of the brain. What’s more, since these areas are also affected by certain medicines and substances such as alcohol, it is of extreme importance to avoid usage of psychoactive substances during Champix treatment and consult your doctor about possible drug interactions if you’re using some other medication alongside Champix.

Common Champix side effects

Side Effect is an unwanted response to a medication taken in normal doses and as such, they are often researched and documented during the clinical trials. While many widely used medicaments display side effects, it is very important that they are carefully documented so that patients can be aware of them and react accordingly if they manifest any after starting the treatment.

The most common documented side effect of Champix therapy is nausea which manifests in roughly 30% of patients who take larger doses and 16% of those who take smaller doses. However, it is often mild and limited to the first period of the treatment after which it usually diminishes and disappears completely. Due to this, nausea leads to stopping of treatment in less than 3% of patients and is thus regarded to be a mild Champix side effect which usually doesn’t need to raise any concerns.

Other Champix side effects which are considered to be rather mild, but less common than nausea manifest in at least 1% of patients in the clinical trials and include headache, constipation, diarrhoea, flatulence, indigestion and upset stomach. Some report sleepiness and slightly changed sense of taste. However, most of the patients experience just a few or none Champix side effects. Furthermore, they are rather mild and usually disappear after the patient’s organism is adjusted to the presence of varenicline. However, if these symptoms get more intensive or don’t pass after a period of up to two weeks, it is highly recommended that you consult your doctor.

Rare Champix side effects

Allergic reaction to Varenicline

Most dangerous, yet very rare side effects of Champix are actually caused by an allergic reaction to varenicline which can be rather severe, although it is very rare. If you start noticing any of these symptoms, it is of vital importance to stop taking medication instantly and contact your doctor. Allergic reaction to varenicline usually manifests through swelling of the face, throat, tongue, lips, gums, eyes, neck, hands, arms, feet, ankles, or lower legs, hoarseness, difficulty swallowing or breathing, rash, swollen or red skin, pain or discomfort in the chest area, one or both arms, back, neck or jaw and shortness of breath. It is very important that you get familiar with all the signs of allergic reaction to varenicline before taking Champix so you can notice these side effects as soon as they manifest. You can find the complete lists of usual symptoms in the special precautions sections of Champix guidelines.

Mood changes

In very rare cases, due to varenicline’s mechanism of action, an unexpected dopamine interaction can occur and lead to mood and behaviour changes. These Champix side effects are extremely rare, but if you start noticing these symptoms, it is imperative to contact your doctor immediately. It is also recommended that you notify your friends and family members of this possibility in advance so they can monitor your behaviour and let you know if there are some noticeable changes in your mood or usual behaviour. Although very rare, depression is a documented side effect of Champix, manifested in a fraction of patients in the clinical trials. Aside from depression, some patients reported anger as a possible Champix side effect, although even more rarely than depression.

Sleep disorders

Another rare Champix side effect is its unwanted impact on sleep patterns. When this does occur, patients usually report very vivid or unusual dreams and sometimes even nightmares. It is suspected that this might be a side effect of the way varenicline interacts with the dopamine through nicotinic receptors. But, despite this, Champix side effects related to sleep disorders are almost exclusively limited to dreams and tend to diminish and completely disappear over the course of the treatment. So, similar to the more common side effects, they are considered to be rather mild and almost never lead to cessation of the treatment.

Long-term Champix side effects

So far, no long-term Champix side effects have been reliably identified although there are some indications that impact on mood can be prolonged even after the stopping of the treatment. A very small fraction of patients reported the feelings of depression and unusual and vivid dreams for weeks after the stopping of the Champix treatment. However, so far, the sample is still extremely small, so there is no reliable data about long-term side effects.

Drug interactions

Due to the unique mechanism of action of varenicline, Champix shouldn’t be combined with other medications that affect central nervous system or dopamine and serotonin receptors. Most of the drugs of this type are used for the treatment of various mental illnesses so if you are using them, you should consult your doctor before starting Champix treatment and ensure that there won’t be any interactions between medications that could cause unwanted side effects. Similar to this, psychoactive substances including alcohol can interact with Champix which can result in very unpleasant and potentially dangerous side effects, so it is of vital importance that you make sure that you don’t drink or consume any other psychoactive substances during Champix treatment.


The high success rate and unprecedented effectiveness of Champix don’t come without certain risks. The clinical trials also helped shed some light on the side effects of this smoking cessation medicine. Most of them stem from the mechanism of action of varenicline. While the drug affects certain neurotransmitters and receptors closely related with dopamine in the brain in order to diminish cravings and withdrawal effects and reduce the pleasure experienced from consuming tobacco products, these same interactions can lead to some rather noticeable and unpleasant side effects. Most of the common ones, such as nausea, upset stomach and constipation are usually mild and temporary so even if they become more prominent, the symptoms will diminish and gradually disappear over the course of the treatment.

Rarer Champix side effects which manifest in only a fraction of patients in the test trials are even more closely connected with the impact of varenicline on dopamine and can lead to rather serious symptoms, including mood and behaviour changes and even depression. So, before starting your Champix treatment, it is imperative to consult your doctor and tell him about any prior conditions or current medications you might be taking that might have an impact on the possibility of some of these side effects manifesting. And if you do start taking Champix, you should be familiar with the possible unwanted symptoms so you can notice them as soon as possible if they manifest. In that case, it is best to immediately contact your doctor and consult with him about the continuance or eventual termination of the treatment.