Mounjaro Side Effects: Common Reactions, Risks And How To Manage Them Safely

Every medication can cause side effects, and Mounjaro is no different. Because of the way it works, it’s common to notice digestive issues, especially when you’re new to it or are increasing your dose.

If you’re just starting your weight management treatment, it’s important to know which symptoms you may experience and how you can manage them.

Here, our experts break down what the most common and serious Mounjaro side effects are. We’ve also included tips on how to relieve discomfort and make your treatment as safe and easy as possible.

Article Summary:

• Mounjaro is the brand name of a weight loss medication that’s injected once a week to help treat obesity, alongside lifestyle changes.
• It contains the active ingredient tirzepatide, which is known as a dual GIP and GLP-1 receptor agonist. This means it mimics two hormones naturally released by the body.
• Mounjaro can cause side effects including nausea, diarrhoea and constipation, but these usually ease off as your body adjusts to the medication.

What is Mounjaro and how does it work?

Mounjaro is a weight loss injection which contains the active ingredient tirzepatide. It works by mimicking two hormones naturally released by your stomach, GIP and GLP-1. It’s used to help with weight loss because it:

• Reduces hunger: Tirzepatide mimics the hormones that tell your brain that you're full.
• Keeps you fuller longer: Tirzepatide slows digestion, helping you stay full between meals.
• Regulates blood sugar: Tirzepatide helps keep blood sugar stable, reducing cravings.
• Silences ‘food noise’: Tirzepatide influences the part of your brain that controls appetite to reduce food focused thoughts. [1]

Mounjaro’s dual action makes it more effective for weight loss than semaglutide (e.g. Ozempic, Wegovy), which only activates GLP-1 receptors. [2] A major study showed that tirzepatide users lost 20.9% of their weight in 72 weeks when combined with diet and exercise changes. [3]

The treatment is injected into your stomach, upper arm or thigh once a week using a pre-filled pen. Your injection site should change each week to avoid skin irritation. You can use the same area, but don’t inject directly into the same spot as the week before.

Common Mounjaro side effects

Side effects are most common when you’re just starting tirzepatide, but tend to improve over time for most people. That’s because your body should gradually adjust to the treatment. The way it slows down your gut can cause gastrointestinal side effects like feeling sick or an upset stomach, but they’re usually mild or moderate symptoms that go away as you get used to it.

The most common side effects of tirzepatide for weight loss are:

• Nausea (up to 33.3%)
• Diarrhoea (up to 23.0%)
• Constipation (up to 17.1%)
• Indigestion (up to 11.3%)
• Vomiting (up to 12.2%)

Other common side effects include: injection site reactions, dizziness, stomach pain, bloating, gas, fatigue, hair loss, and increased heart rate. [4]

You may also have a decreased appetite due to the drug helping to slow your digestion. If you’re experiencing side effects like constipation and indigestion, you can manage this by maintaining a healthy diet that’s rich in fibre, staying hydrated and doing regular exercise.

Changing your diet can also help alleviate vomiting and diarrhoea. Continue reading for more detailed information on that below.

It's important that you monitor any side effects and consult your clinical team if they continue or become worse. Seek medical help if you experience severe abdominal pain, trouble breathing, swelling, or persistent vomiting. [2]

Who can use Mounjaro?

To find out if Mounjaro is the right treatment for you, you’ll need to have a consultation with a clinician to see if you’re eligible. This includes a review of your health history and your weight loss goals.

Tirzepatide can be prescribed to treat obesity in adults with a body mass index (BMI) of 30 or higher, or a BMI of 27 or higher if you also have a weight-related health condition like high blood pressure, type 2 diabetes, or high cholesterol.

Who shouldn’t use Mounjaro?

Weight loss injections aren’t for everyone. As well as having a clinical assessment to check your eligibility, it’s important to know if any of the following apply to you.

Mounjaro isn’t suitable for you if you’re currently pregnant or breastfeeding. You should stop taking tirzepatide at least one month before a planned pregnancy, as that’s how long it takes to leave your system completely. Remember, tirzepatide may make your oral birth control less effective by lowering hormone levels in your blood. Use condoms or switch to a non-oral method like an IUD to stay protected against pregnancy. [5]

Do not use Mounjaro if you have a personal or family history of a type of thyroid cancer called medullary thyroid carcinoma (MTC), or if you have Multiple Endocrine Neoplasia syndrome type 2 (MEN 2). [2]

You also shouldn’t take Mounjaro if you’re allergic to tirzepatide or any other ingredients in the medication. Severe allergic reactions are rare, but you should seek immediate medical attention if you notice a skin rash, throat swelling or difficulty breathing.

If you have pre-existing conditions like kidney issues, a history of pancreatitis or gallbladder disease, or severe digestive problems, speak to your doctor before starting Mounjaro.

Serious side effects and warnings

Other rare side effects of Mounjaro may include pancreatitis, a potentially life-threatening condition requiring immediate emergency care. It can also cause gallbladder problems. [2]

For more information about side effects, you can view the Mounjaro patient information leaflet here.

When to seek medical attention

Seek medical advice if you notice any symptoms of pancreatitis. This includes sudden, severe stomach pain, feeling or being sick and a high temperature of 38 °C or more (fever). [7]

You should also seek urgent medical advice if you experience symptoms of an allergic reaction or anaphylaxis, such as difficulty breathing, a skin rash or swelling of the lips, face or tongue.

Experiencing severe pain underneath your right ribcage (that may spread to your shoulder or back) can indicate acute gallbladder problems. This may be accompanied by vomiting, a high temperature or a rapid heartbeat and requires immediate medical attention.

You may be at a higher risk of getting low blood sugar if you take Mounjaro with another medicine such as insulin. Signs of low blood sugar include dizziness, lightheadedness, sweating, confusion, headache, blurred vision, slurred speech, and feeling shaky or jittery. Seek medical attention if you experience any of these symptoms. [2]

Managing side effects of Mounjaro

Consistency is key when taking Mounjaro, so it’s important that you’re comfortable with your dosage. Here are some tips you can follow to keep side effects at bay:

Diet

To reduce nausea and vomiting, eat small portions, limit fatty and spicy foods, avoid strong smells and eat crackers, mint, or ginger-based foods or drinks. [6] For constipation, include high-fibre foods and plenty of fluids in your diet. Stay hydrated and rest, to help manage diarrhoea. [8]

Medication

Over-the-counter antacids may help with heartburn. [8] If side effects are severe or persistent, contact our clinical team for support.

Dosage and storage

If you miss a dose, take it as soon as you remember within four days. If it's been more than four days, skip it and take the next dose as usual on your scheduled day. Mounjaro can be stored at room temperature for up to 30 days (not above 30 °C) after removal from the fridge. [1].

Remember, most side effects tend to resolve as your body adjusts to the medication. If they’re severe or persistent, contact your clinical team for support.

Got a question about side effects?

Speak to our dedicated clinical team for any extra support. If you haven’t started your weight loss journey and want to learn more, find out what treatment is suitable for you on our website.

References

1. Mounjaro KwikPen 10mg solution for injection in pre-filled pen - Summary of Product Characteristics (SmPC) - (emc) [Internet]. www.medicines.org.uk. Available from: https://www.medicines.org.uk/emc/product/15484/smpc#gref
2. Mounjaro: Uses, Dosage, Side Effects & Warnings [Internet]. Drugs.com. Available from: https://www.drugs.com/mounjaro.html
3. Jastreboff AM, Aronne LJ, Ahmad NN, Wharton S, Connery L, Alves B, et al. Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine [Internet]. 2022 Jun 4;387(3):205–16. Available from: https://www.nejm.org/doi/full/10.1056/NEJMoa2206038
4. Mounjaro KwikPen 2.5mg solution for injection in pre-filled pen - Summary of Product Characteristics (SmPC) - (emc) [Internet]. www.medicines.org.uk. Available from: https://www.medicines.org.uk/emc/product/15481/smpc#gref
5. Skelley JW, Swearengin K, York AL, Glover LH. The impact of tirzepatide and glucagon-like peptide 1 receptor agonists on oral hormonal contraception. Journal of the American Pharmacists Association. 2023 Nov 1;64(1).
6. Mounjaro Side Effects: Common & Serious Risks [Internet]. Drugwatch.com. 2024. Available from: https://www.drugwatch.com/drugs/mounjaro/side-effects/
7. NHS. Acute pancreatitis [Internet]. nhs.uk. 2022. Available from: https://www.nhs.uk/conditions/acute-pancreatitis/
8. Mounjaro Side Effects: What They Are and How to Manage Them [Internet]. Healthline. 2023. Available from: https://www.healthline.com/health/drugs/mounjaro-side-effects