Warfarin, Insulin, and More: 6 Varenicline Interactions to Watch Out For

Varenicline (previously available as brand-name Chantix) is a prescription-only medication that helps people quit smoking. Varenicline doesn’t contain nicotine. Instead, it works by attaching to parts of the brain where nicotine attaches. It also increases the levels of a chemical called dopamine. These actions make smoking less enjoyable, reduce cravings, and minimize nicotine withdrawal symptoms.

Like all medications, varenicline can have side effects. It also has drug interactions to consider. Below, we’ll focus on six notable varenicline interactions and discuss steps you can take to prevent them. We’ll also cover what to do if you’re experiencing symptoms of an interaction.

Varenicline interactions summary

The following medications and substances can interact with varenicline:

• Warfarin (Jantoven)

• Diabetes medications, such as insulin

• Theophylline (Theo-24, Elixophyllin)

• Alcohol

• Nicotine replacement therapy, such as nicotine patches (Nicoderm CQ, Habitrol)

• Medications that increase the risk of seizures, such as bupropion (Wellbutrin SR, Wellbutrin XL)

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This isn’t a comprehensive list. So be sure to let your prescriber know about all of the prescription and over-the-counter products you take before starting treatment with varenicline.

1. Warfarin

Warfarin is a blood thinner that helps prevent blood clots. If you take warfarin, you may need regular blood tests to check your international normalized ratio (INR). This is a measure of how quickly your blood clots. It helps your prescriber determine the right warfarin dosage for you. A warfarin dosage that’s too high can cause bleeding. A warfarin dosage that’s too low can be ineffective.

Varenicline and warfarin don’t interact directly. Rather, warfarin’s interaction is with smoking itself. Certain chemicals in cigarettes affect how your liver metabolizes (breaks down) warfarin. These chemicals, called polycyclic aromatic hydrocarbons (PAHs), speed up how quickly warfarin leaves your body. This can decrease warfarin levels in your blood, making the medication less effective. 

PAHs are created when you light a cigarette and smoke is created. However, PAHs are also in smokeless tobacco products, though to a lesser extent. This includes moist snuff, which is also known as “dip.” E-cigarettes, often called vapes, can also create PAHs. 

So how does this relate to varenicline? When you quit smoking (or using other tobacco products), warfarin may leave your body at a slower rate than it did when you smoked. As a result, warfarin levels in your blood may increase, which can raise your risk of bleeding. Warfarin is a narrow therapeutic index drug, so even small increases in warfarin levels can cause bleeding. 

If you’ve recently quit smoking or plan to, let your prescriber know. They may want to check your INR more frequently in the weeks that follow you quitting. And they may need to adjust your warfarin dosage to keep your INR within a safe range.

Keep in mind that quitting smoking is always a positive lifestyle choice. For one thing, smoking increases your risk of blood clots. So quitting can lower your risk for clots, among many other health benefits. 

2. Diabetes medications, such as insulin

Insulin is a natural hormone that controls your blood glucose (sugar). It moves glucose from your blood into your cells to be used as energy. For people with diabetes, their bodies either don’t have enough insulin or don’t use it properly. This can increase blood glucose levels and cause hyperglycemia. 

While varenicline doesn’t directly affect insulin levels, smoking can. The nicotine in cigarettes makes it harder for your body to use insulin properly, which can lead to hyperglycemia. You may need a higher dosage of diabetes medications if you smoke cigarettes or use other tobacco products, such as chewing tobacco or e-cigarettes. Examples of commonly prescribed diabetes medications include metformin, insulin, and semaglutide (Ozempic).

Quitting smoking with the help of varenicline can improve your body’s response to insulin. While this is a good thing overall, it may cause hypoglycemia (when your blood glucose drops too low). So, if you’ve recently quit smoking or plan to, let your diabetes care team know. They may ask you to monitor your blood glucose levels at home more often. And they may lower the dosage of your diabetes medications to prevent hypoglycemia.

Keep in mind that quitting smoking has additional benefits for people with diabetes. As mentioned, nicotine can increase blood sugar, making your diabetes harder to control. Smoking also increases your risk of complications from diabetes, such as kidney and heart disease. So quitting smoking, or using other tobacco products, can greatly improve your health.

3. Theophylline

Theophylline is a medication that’s used to treat asthma and chronic obstructive pulmonary disease (COPD). It’s a bronchodilator, which means it helps relax the airways, making it easier to breathe. 

PAHs in tobacco products can increase how quickly your body metabolizes theophylline. So you may need a higher dosage of theophylline if you smoke cigarettes or use other tobacco products. 

If you’re taking varenicline and quit using tobacco products, your body may metabolize theophylline more slowly. This can cause higher theophylline levels in your blood. Theophylline is a narrow therapeutic index drug, so even small increases in blood levels can be dangerous. Examples of side effects caused by high theophylline levels, or theophylline toxicity, include nausea, vomiting, and headaches. 

If you quit smoking or using other tobacco products, let your prescriber know. They’ll likely check your theophylline levels and adjust your dosage as needed. 

Keep in mind that quitting smoking has major benefits for your lungs. Smoking damages your lungs and increases your risk for lung infections. Quitting can make breathing easier, improve your quality of life, and decrease your risk of asthma or COPD flare-ups.  

4. Nicotine replacement therapy

To help you quit smoking, a healthcare professional may recommend combining varenicline with nicotine replacement therapy (NRT). NRT works by giving your body a small, controlled dose of nicotine, without the harmful chemicals found in tobacco products. This helps reduce cravings and nicotine withdrawal symptoms. NRT comes in various forms, including nicotine patches (Nicoderm CQ, Habitrol CQ), gum (Nicorette), and lozenges (Nicorette). 

Varenicline and NRT have some common side effects, including nausea, stomach upset, and sleep issues. Headaches and dizziness are also possible with both. Combining varenicline with NRT can make these side effects more likely.

Still, combining varenicline and NRT can be an effective way to quit smoking. And not everyone experienced increased side effects. Don’t combine these products without consulting a healthcare professional, however. They can help you adjust your medication regimen in a way that minimizes side effects. 

If you do experience side effects with this combination, let your prescriber know. But don’t stop varenicline or NRT without talking to them first. Stopping smoking cessation medications early can increase your risk of returning to smoking.

5. Alcohol

Varenicline can affect how you react to alcohol. Combining varenicline and alcohol can increase feelings of intoxication, making you feel more inebriated than you usually would. Unusual or aggressive behaviors and short-term memory loss have also been reported.

Before taking varenicline, talk to your prescriber about whether it’s OK to drink alcohol. They may recommend avoiding alcohol entirely. Or they may recommend avoiding it until you see how varenicline affects you.

6. Medications that increase seizure risk

Several medications can raise your risk of seizures. Examples include bupropion, Adderall (mixed amphetamine salts), and carbamazepine (Tegretol). If you take any of these medications, or have a history of seizures, varenicline may not be the best treatment option for you. While rare, there have been some reports of seizures with varenicline use. Seizures typically within the first month of treatment with varenicline.

If you have a seizure while taking varenicline, stop taking the medication and seek medical help right away. Before you start taking varenicline, make sure your prescriber knows about your full health history and all the medications you take. This includes over-the-counter medications or supplements.

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Written by:

Arnisha Carter, PharmD, BCPS

Arnisha Carter, PharmD, BCPS, is a licensed pharmacist in Florida. She’s worked in the pharmacy industry for more than 10 years and began writing for GoodRx in 2024.

Edited by:

Amy B. Gragnolati, PharmD, BCPS

Amy Gragnolati, PharmD, BCPS, is a pharmacy editor for GoodRx. Amy currently holds her pharmacist license in Georgia and California.

Reviewed by:

Daphne Berryhill, RPh

Daphne Berryhill, RPh, has two decades of experience as a clinical pharmacist. She spent most of her career in the Chicago area practicing in-home infusion.

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