Does Mounjaro Cause Atrial Fibrillation (AFib)?

Is AFib listed as a Mounjaro side effect in the UK?

The short answer is no. The Mounjaro Summary of Product Characteristics (the document that governs how the medicine is prescribed and used in the UK) does not include atrial fibrillation in its list of known adverse reactions. This document is compiled from the full SURMOUNT clinical trial programme, which involved thousands of adults over periods of up to 72 weeks, and cardiac events were tracked throughout.

The side effects that do appear are predominantly gastrointestinal: nausea, diarrhoea, constipation, indigestion, and similar complaints that tend to peak in the first weeks of treatment or after a dose increase. You can read the NHS patient-level summary of tirzepatide's side effects on the NHS website, which is updated as new safety data emerge and is the clearest plain-English source available.

None of this means a connection is impossible in every individual. If you've experienced palpitations on Mounjaro, your prescriber needs to know — but the evidence to date does not establish tirzepatide as a cause of AFib.

Why do people connect Mounjaro and AFib in the first place?

It's a reasonable question, not a fringe one. There are a few reasons it keeps coming up. First, GLP-1 and GIP receptor agonists do have modest effects on heart rate, a small rise in resting pulse is well-documented and appears in the prescribing information. That's distinct from an arrhythmia, but it's enough to make people with existing heart concerns pay attention to every beat.

Second, the people taking Mounjaro often have elevated cardiovascular risk profiles. Obesity, high blood pressure, type 2 diabetes and sleep apnoea all raise the background probability of AFib independently. When someone starts a new medicine and later develops a heart-rhythm problem, the timing feels causal even when it isn't.

Third, rapid weight loss through any method can temporarily alter electrolyte balance and cardiac load. Understanding how Mounjaro causes weight loss matters here: it works by reducing appetite rather than through anything that directly targets cardiac tissue. Still, as the body changes composition, monitoring matters.

Semaglutide (Wegovy) has actually shown cardiovascular benefit in large trials, reducing the risk of major events in people with obesity and established heart disease. Whether tirzepatide carries a similar benefit is being studied; results are anticipated but not yet published in full.

What should you do if you notice palpitations or an irregular pulse?

A practical check worth doing: place two fingers lightly on the inside of your wrist, just below the base of your thumb, and count beats for 30 seconds. If the rhythm feels irregular rather than simply fast, that's worth flagging. An irregular pulse is one of the more reliable self-detected signs of AFib, and it takes under a minute to check.

If you're already on Mounjaro and you notice a racing heart, palpitations or an irregular beat that's new or persistent, contact your GP or prescriber the same day, don't wait for your next review. If you feel chest pain, breathlessness or faintness alongside the palpitations, call 999. The MHRA's Yellow Card scheme also lets patients report suspected medicine side effects directly, which helps build the national safety picture over time.

The full Mounjaro treatment page at nume sets out how our prescribers review each patient's health history before treatment starts, including any existing cardiovascular conditions. People with a documented history of arrhythmia or significant heart disease are assessed individually, not automatically excluded, but looked at carefully.

Does existing AFib stop you from using Mounjaro?

Not automatically. Having atrial fibrillation is not listed as a contraindication in the Mounjaro SmPC, and many people living with well-managed AFib have metabolic risk factors that tirzepatide could address. The clinical picture matters: how well your rhythm is controlled, what anticoagulation or rate-control medicines you're already taking, and whether your cardiologist or GP has any specific concerns.

What the prescribing process at a regulated service involves is a proper read of your answers, not a checkbox exercise. A real clinician looks at the full picture, including any existing heart conditions, medicines that might interact, and whether your GP is aware of the plan. We notify GPs in line with GPhC guidance as standard. You can find more about how that process works on the about us page.

There are other side-effect questions that come up alongside cardiac ones. Patients sometimes ask about whether Mounjaro causes gas and similar GI concerns, or less obvious effects like changes to skin during treatment, and some also read up on whether Mounjaro can cause erectile dysfunction before they start. These are well worth reading before you start, so there are no surprises.

If you'd like to discuss your specific cardiac history with our prescribers, the right place to start is a free consultation. There's no obligation, and no shortcuts, just a clinician reading your answers the same day. Treatment is available in several formats, including if you're interested in the tirzepatide ampoule option. Start your free consultation here.