Mounjaro and Atrial Fibrillation (October 2026)

01 · Your situation

Does Mounjaro cause atrial fibrillation?

Mounjaro (tirzepatide) is a prescription-only medicine with a Summary of Product Characteristics and a patient information leaflet that list its recognised side effects.¹¹³

Atrial fibrillation is not named in the UK patient leaflet, but a fast pulse is listed as a common side effect, and a symptom not being listed does not mean it is nothing. It simply means the cause may lie elsewhere, and it still deserves assessment.

02 · Next step

What to do if you have atrial fibrillation or palpitations

If you already have atrial fibrillation, say so in your consultation, along with every medicine you take. Mounjaro is not suitable for everyone, and our prescriber needs your full medical history and medicine list to decide.¹ Your cardiologist or GP knows your rhythm history and treatment, so involve them before starting.

If you are already taking Mounjaro and notice a racing, fluttering or irregular heartbeat, get medical help straight away rather than waiting for your next dose or review. If palpitations come with chest pain, severe breathlessness or fainting, call 999 or go to A&E.

03 · Reading the label

Where to check Mounjaro's side effects

Both are published on the electronic medicines compendium, and the patient information leaflet comes with your pen.¹¹³

For symptoms that no one has clearly linked to the medicine, the right approach is plain: discuss the new symptom with your prescriber, and report it through the Yellow Card scheme so regulators can see patterns.¹⁹ Reporting is not an admission that the medicine caused it. It is how safety signals are spotted.

04 · Indirect effects to know

Indirect effects: dehydration and general wellbeing

Severe vomiting or diarrhoea can cause dehydration, which can strain the kidneys, so keep fluids up and get medical help straight away if you cannot keep fluids down.¹ Feeling unwell and drained is a reason to get in touch, whatever the cause.

If you have a racing heart, get medical help straight away; it is not the same as a diagnosed rhythm disorder. Only a clinician, usually with an ECG, can tell the difference.

If you have a history of other symptoms such as swelling, see our guide on swollen ankles on Mounjaro. For another rhythm question, read our page on Mounjaro and SVT.

05 · Heart health and weight

Weight loss and wider heart health

In trials, weight loss on tirzepatide came with average improvements in waist size, blood pressure, cholesterol and blood-sugar measures.² These are trial-average secondary outcomes, not promises, and they do not tell us what happens to heart rhythm specifically.

In SURMOUNT-1, people taking tirzepatide alongside diet and exercise lost on average 15% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% with lifestyle changes alone. Individual results vary, and treatment works alongside a reduced-calorie diet and increased physical activity.² Weight management is one part of looking after your heart, and a cardiologist can advise how it fits with your own condition.

06 · Who should be cautious

Who needs extra prescriber care

Mounjaro should not be used if you are allergic to tirzepatide or its ingredients, or during pregnancy or while trying to conceive. Mounjaro must not be used during pregnancy, and it should be stopped at least one month before trying to conceive. If you become pregnant during treatment, stop the medicine and tell your prescriber promptly. Your prescriber will take extra care, or may recommend against treatment, if you have a history of pancreatitis, severe gut or digestive disease, diabetic retinopathy or gallbladder problems.¹

If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar, such as shakiness, sweating or confusion, as those medicines may need adjusting.¹

Serious reactions are uncommon but urgent. Seek immediate medical help for severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing. Our page on anaphylaxis and Mounjaro covers the allergic signs.

07 · Other symptoms people ask about

Other symptoms people ask about

Questions about heart rhythm often sit alongside other worries. If you are wondering about unusual effects, our related guides cover photosensitivity, neuropathy, erectile problems and rhabdomyolysis.

Whenever a symptom is new, persistent or frightening, tell a clinician. Do not stop or change anything on your own without advice.

08 · Ordering with a heart condition

How ordering works if you have a heart condition

Our process starts with a free online consultation, a short health questionnaire with no charge and no obligation. A GPhC-registered prescriber reviews it, not a scoring script, and if the prescriber decides the treatment is not suitable, the order is refunded automatically in full.

Be specific about your heart diagnosis, procedures, and medicines. Repeat orders re-check suitability, and our aftercare includes monthly reviews and side-effect support, so tell us if your health changes. Delivery is free, next-day and tracked, and same-day dispatch applies when ordered by 3pm.