Is Mounjaro safe?

Yes, when Mounjaro is prescribed and used correctly, it is a safe and effective weight loss treatment. Your doctor will consider your medical history, including past and current conditions and medications, and your body mass index (BMI) when prescribing Mounjaro.

Mounjaro may not be safe if you take it without a prescription, and as with any medication, there is a chance of serious side effects. Most Mounjaro side effects are mild and go away over time but being aware of side effects and reporting them to your doctor can help keep you safe during your treatment.

The Mounjaro dosage schedule is also in place to introduce the active ingredient (tirzepatide) into your body slowly, reducing the chance of side effects. You can stay on a lower dose for longer if you’re worried about side effects, and are still seeing the benefits of the medication and losing weight.

Mounjaro has been thoroughly tested and approved for use by the MHRA for both type 2 diabetes and weight loss.

Type 2 diabetes trials

Mounjaro was reviewed closely in 5 clinical trials, known as SURPASS 1 to 5, involving 6,263 patients. The study results found that Mounjaro significantly decreased blood sugar levels for up to 2 years. During these trials, it was also found that Mounjaro helped people to lose weight.

Weight loss trials

Mounjaro was then studied in the SURMOUNT 1 and 2 clinical trials as a weight loss medication. This involved 3,477 overweight or obese patients with and without diabetes. The results found significant reductions in body weight compared to a placebo, regardless of the patient's age, race, ethnicity, blood sugar levels, and baseline BMI.

Side effects

For weight management, the most common Mounjaro side effects noted in clinical trials were those affecting the gastrointestinal (GI) tract: nausea, vomiting, diarrhoea and constipation. These were mostly mild (61%) to moderate (34.4%) and usually seen after the dose was increased and improved over time. 2.1% of patients stopped taking tirzepatide due to GI side effects.

Mounjaro benefits

As Mounjaro can help reduce your BMI, it can lead to a reduction of developing weight-related medical conditions, or an improvement in conditions you already have, including:

• diabetes
• cardiovascular disease
• high blood pressure
• PMOS
• high cholesterol

The most common Mounjaro side effects (may affect more than 1 in 10 people) are:

• nausea (feeling sick)
• vomiting (being sick)
• diarrhoea
• constipation
• stomach pain
• low blood sugar when used for type 2 diabetes (not weight loss) with medications containing insulin or sulphonylurea

Mounjaro side effects are usually not severe and are most likely to happen when starting treatment or increasing your dose. If you do get side effects like vomiting and diarrhoea, it’s very important to stay well hydrated, as dehydration can decrease kidney function. If they do not improve or get worse, speak to a doctor for advice.

Although stomach pain, constipation, and vomiting are very common in people treated for weight loss, they are listed as common side effects if you take Mounjaro for type 2 diabetes. Knowing what to eat on Mounjaro may reduce the likelihood and severity of GI side effects. For example, processed, fatty, or greasy foods can make them worse, while nutritious, healthy, and small meals could improve them.

It’s also important not to reduce your food intake too much, as having an empty stomach while taking Mounjaro may increase these side effects. Although Mounjaro is focused on weight loss and calorie reduction, eating too little is also unsafe and could cause nutritional deficiencies and other health problems, including hair loss.

Serious side effects of Mounjaro

Mounjaro can cause low blood sugar (hypoglycaemia), and how common this is depends on whether you’re taking other type 2 diabetes medications at the same time. For example, it's very common when taken with sulphonylurea or insulin. If you’re using these medications, the dose may need to be lowered, so it’s important to speak to your doctor or diabetes specialist.

Low blood sugar can be treated at home if you are diabetic and know how to, but it should still be reported to your doctor. However, low blood sugar can also be a medical emergency and taking too much Mounjaro increases the risk of this side effect, even if you are not diabetic. Signs of low blood sugar include:

• headache
• drowsiness
• weakness
• dizziness
• intense hunger
• confusion
• irritability
• fast heartbeat
• sweating

Uncommonly (may affect up to 1 in 100 people), Mounjaro can cause:

• an inflamed pancreas (acute pancreatitis) – symptoms include long-lasting and severe pain in the stomach and back, with or without vomiting
• cholecystitis (infection of the gallbladder) – symptoms include sharp, long-lasting pain and tenderness in the upper right abdomen, fever, nausea, vomiting, and sweating

Rarely (may affect up to 1 in 1,000 people), Mounjaro can cause a severe allergic reaction (anaphylaxis or angioedema). Symptoms include:

• problems breathing
• swelling of the face and lips
• difficulty swallowing

Mounjaro risks are most likely if you have a medical condition or take a medication that may interact with it, but your doctor will make sure this isn’t an issue before prescribing it.

Mounjaro risks may be increased if you:

• do not take it properly, such as doubling your dose or taking it more than once a week
• do not obtain a prescription from a doctor
• choose to buy Mounjaro pens from an unregulated website, as the pens could be fake

If you use more Mounjaro than you have been prescribed, talk to your doctor or call 111 straight away. Taking too much may cause low blood sugar (hypoglycaemia), nausea, and vomiting.

Who should not take Mounjaro for weight loss?

You should not use Mounjaro if you are:

• allergic to any ingredients in the medication
• pregnant
• planning to get pregnant
• breastfeeding

Mounjaro may also not be appropriate if you:

• have severe problems with food digestion, such as severe gastroparesis (a disorder that slows or stops the movement of food from your stomach to your small intestine)
• have ever had pancreatitis – Mounjaro can also cause this
• have a problem with your eyes, such as diabetic retinopathy or macular oedema – Mounjaro may make your symptoms worse
• are diabetic and are using sulphonylurea or insulin – low blood sugar can occur so your doctor may need to change your dose of these medications
• are due to have surgery under anaesthesia

If you’re not sure, speak to your doctor.

To use Mounjaro safely, you need to know how to inject it, when to increase your dosage, and how to store it. Your doctor will provide full instructions on this, and the information is also available to reference at any time in your patient information leaflet and user manual that comes with your Mounjaro pen.

Injecting Mounjaro safely

The 3 main injection sites are your abdomen, upper arms (if you can get someone to do it for you), and upper legs (thighs). Find out more about injecting Mounjaro safely in our Mounjaro injection guide.

Following the Mounjaro dosage schedule

The Mounjaro dosage schedule involves increasing your dose every 4 weeks until you reach a maintenance dose that works for you. The starting dose is always 2.5mg once a week, and the maximum maintenance dose is 15mg.

You can also use a maintenance dose of 5 or 10mg. You may want to stay on a lower dose of tirzepatide because of side effects, or because you’re happy with your weight loss results and do not want to take a higher dose. If you change your mind at any point, you can increase your dose as long as it’s been at least 4 weeks on your current dose.

"It’s very important that you stick to the dosage schedule, as it’s there for your safety. In a 2025 study on Mounjaro, incorrect dose administration was the biggest reason for an increase in adverse effects, rising from 1,248 in 2022 to 9,800 in 2024. Mounjaro doses are a widely discussed topic online, especially the Mounjaro ‘golden’ or ‘fifth dose’, in which people inject a small amount of remaining liquid as an extra dose. This small amount of liquid left in the pen is for priming and should not be used, and taking it means you’re taking more than your prescribed dose and putting yourself at risk of extra side effects if you take it alongside your normal doses.

Our doctor guide on Mounjaro dosages explains how and why you’ll increase your dose, and what to do if you get side effects.”

– Dr Brenda Nomann, Medical Editor

Storing Mounjaro correctly

Improper storage of Mounjaro could cause the medication to no longer be usable, effective, or safe. When you receive your Mounjaro pen, it will come in cool packaging. Unopened Mounjaro pens must be stored in the fridge between 2 and 8°C. They cannot be frozen.

Once opened, you can store them in or out of the fridge, as long as the temperature does not exceed 30°C. For more detailed information on storing Mounjaro, see our Mounjaro storage guide.

Can Mounjaro be dangerous for some people?

Yes. Mounjaro, like any other medication, can be unsafe for some people to take. That is why it’s a prescription medication, and you need to speak to a doctor before you take it. Mounjaro could be dangerous for people who:

• have an allergy to any of the active ingredients
• are pregnant or breastfeeding
• have severe problems with food digestion
• have ever had pancreatitis
• have a problem with their eyes
• are diabetic and are using sulphonylurea or insulin
• are due to have surgery under anaesthesia

Is Mounjaro safe for people with type 2 diabetes?

Yes, Mounjaro is safe for people with type 2 diabetes and is also a type 2 diabetes medication. However, if you take other diabetes medications, the dosages of these may need to be changed so that your blood sugar is not lowered too much. Mounjaro for diabetes can only be prescribed by a diabetes doctor. If you want to take Mounjaro for weight loss and have type 2 diabetes, speak to your doctor first.

Which one is safer, Mounjaro or Wegovy?

Wegovy safety data and Mounjaro safety data are quite similar. Most side effects are the same because they work in the same way. The most reported side effects of Wegovy and Mounjaro are GI-related. In studies:

• 70.8% of people reported GI side effects while taking Wegovy 7.2mg, and 3.2% stopped treatment because of this
• 55.6 % of people reported GI side effects while taking Mounjaro 15mg, and 4.3% stopped treatment because of this

Is Mounjaro from online pharmacies safe?

Yes, as long as the online pharmacy is regulated and registered with the correct UK bodies, such as the General Pharmaceutical Council. If you cannot find this information on a website, or you’re promised medication without a BMI and medical health check, or for very cheap, then the treatment is not safe, and may even be fake.

Is Mounjaro safe for heart patients?

Yes, Mounjaro is safe for heart patients and your doctor may recommend using a weight loss medication like Mounjaro if you’re overweight to help improve your condition or reduce the risk of serious events. As well as reducing BMI, which can improve heart health, studies have shown that tirzepatide can:

• lower the risk of inflammatory markers associated with cardiovascular events (like a heart attack)
• lower the risk of death or worsening heart failure and improve health in patients with heart failure with preserved ejection fraction

Is Mounjaro safe for kidneys?

Mounjaro is safe for kidneys in most cases, although if you lose a lot of fluids because of vomiting, nausea, or diarrhoea, it may lead to a decrease in kidney function. It’s important to stay hydrated and speak to your doctor if you’ve got severe symptoms, especially if you have kidney problems.

Mounjaro also contains benzyl alcohol, so you should ask your doctor for advice on taking it if you’ve got liver or kidney disease. Large amounts of benzyl alcohol can build up in your body and may cause metabolic acidosis.

Is Mounjaro safe while pregnant?

No, Mounjaro is not safe while pregnant or if you’re trying to become pregnant. This is because it’s not known how much of the active ingredient can be passed to an unborn baby or what effect this could have.

When using Mounjaro alongside an oral contraceptive pill, the manufacturer recommends using another form of contraception, such as a condom, for 4 weeks after starting Mounjaro, and for 4 weeks after each dose increase. Some side effects, like severe vomiting or diarrhoea, could affect how well your contraceptive pill works, especially if you vomit soon after taking it.

Is Mounjaro safe for long-term weight loss?

Yes, Mounjaro is safe for long-term weight loss and can be taken for years with a doctor’s supervision. Clinical studies followed patients for 72 weeks when taking Mounjaro and monitored their week-by-week weight loss results and side effects to make sure it was both safe and effective long-term.

You might need to stop taking Mounjaro if you get side effects that do not go away or are severe. Your doctor will also decide whether to stop treatment if you’ve been taking your maintenance dose for 6 months and have not lost at least 5% of your starting weight.