Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist licensed in the UK for type 2 diabetes and chronic weight management. Whilst clinical trials demonstrate significant efficacy in glycaemic control and weight reduction, understanding the cons of taking Mounjaro is essential for informed treatment decisions. Potential disadvantages include troublesome gastrointestinal side effects, serious risks such as pancreatitis and gallbladder disease, high cost, limited NHS accessibility, and unsuitability for certain patient groups including pregnant women and those with severe gastrointestinal disease. This article provides a balanced, evidence-based overview of the drawbacks associated with Mounjaro therapy to support shared decision-making with healthcare professionals.

What Is Mounjaro and How Does It Work?

Mounjaro (tirzepatide) is a prescription medicine licensed in the UK for the treatment of type 2 diabetes mellitus and, more recently, for chronic weight management in adults with obesity or overweight with weight-related comorbidities. It is administered as a once-weekly subcutaneous injection, starting at 2.5 mg and typically escalating in 2.5 mg steps every 4 weeks to a maintenance dose as tolerated. Mounjaro represents a novel class of medication known as a dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist.

The mechanism of action involves mimicking two naturally occurring incretin hormones that regulate blood glucose and appetite. By activating both GIP and GLP-1 receptors, Mounjaro enhances insulin secretion in response to elevated blood glucose levels, suppresses inappropriate glucagon release, slows gastric emptying, and promotes satiety. This dual action contributes to improved glycaemic control in people with type 2 diabetes and facilitates weight loss by reducing appetite and caloric intake.

Whilst Mounjaro has demonstrated significant efficacy in clinical trials—with substantial reductions in HbA1c and body weight—it is important to recognise that all medicines carry potential disadvantages. Understanding the cons of taking Mounjaro is essential for informed decision-making. These include a range of side effects, some of which can be troublesome or serious, as well as practical considerations around cost, accessibility, and suitability for certain patient groups. The slowing of gastric emptying may also affect the absorption of other oral medicines. This article provides a balanced, evidence-based overview of the potential drawbacks associated with Mounjaro therapy.

Common Side Effects of Mounjaro

The most frequently reported adverse effects of Mounjaro are gastrointestinal in nature, affecting a significant proportion of users, particularly during the initial weeks of treatment or following dose escalation. These include:

• Nausea – often the most common complaint, which may be persistent in some individuals

• Diarrhoea – can range from mild to moderate and may affect daily activities

• Vomiting – occurs less frequently than nausea but can be distressing

• Constipation – some patients experience the opposite gastrointestinal effect

• Abdominal pain or discomfort – cramping or bloating may occur

• Reduced appetite – whilst therapeutically beneficial for weight loss, this can be unpleasant for some

• Dyspepsia – indigestion symptoms may occur

• Gastro-oesophageal reflux – acid reflux can be troublesome

These side effects are related to the drug's mechanism of slowing gastric emptying and are typically dose-dependent. Many patients find that symptoms improve over time as the body adjusts to the medication. Strategies such as slower dose titration, staying well-hydrated, and eating smaller, low-fat meals may help manage these effects. However, for some individuals, gastrointestinal symptoms remain troublesome enough to warrant dose reduction or discontinuation.

Other common side effects include injection site reactions (redness, itching, or swelling), fatigue, headache, and dizziness. There is also a risk of hypoglycaemia (low blood sugar), particularly when Mounjaro is used in combination with other glucose-lowering medications such as sulphonylureas or insulin. Dose adjustments of these medications may be necessary to minimise this risk. Patients should be counselled on recognising symptoms of hypoglycaemia—such as tremor, sweating, confusion, and palpitations—and the importance of monitoring blood glucose levels as advised by their healthcare team.

Importantly, Mounjaro may reduce the effectiveness of oral contraceptives, particularly during the first 4 weeks after starting treatment and for 4 weeks after each dose increase. Women using oral contraceptives should consider using a non-oral method or adding a barrier method during these periods.

Serious Risks and Precautions

Whilst most side effects of Mounjaro are mild to moderate, there are serious risks that require careful consideration and monitoring. One of the most concerning is acute pancreatitis (inflammation of the pancreas). Patients should be advised to seek urgent medical attention if they experience severe, persistent abdominal pain that may radiate to the back, often accompanied by nausea and vomiting. If pancreatitis is suspected, Mounjaro must be discontinued and not restarted.

Gallbladder disease, including cholelithiasis (gallstones) and cholecystitis (gallbladder inflammation), has been reported in patients taking GLP-1 receptor agonists, likely related to rapid weight loss. Symptoms such as right upper quadrant pain, fever, or jaundice warrant prompt investigation.

Animal studies have shown an increased incidence of thyroid C-cell tumours with GLP-1 receptor agonists. Whilst the relevance to humans is unknown, patients should be counselled to report symptoms such as a neck lump, hoarseness, or difficulty swallowing. This is a precaution rather than a formal contraindication in the UK.

Other serious considerations include acute kidney injury, particularly in patients who become dehydrated due to gastrointestinal side effects. Patients with pre-existing diabetic retinopathy, especially those on insulin, may experience temporary worsening of retinopathy with rapid improvements in glycaemic control and should have appropriate eye monitoring. Hypersensitivity reactions, including anaphylaxis and angioedema, though rare, have been reported and necessitate immediate discontinuation.

Mounjaro is not recommended in patients with severe gastrointestinal disease, including severe gastroparesis, as it further slows gastric emptying. The delayed gastric emptying may also affect the absorption of other oral medicines, which may be clinically significant for drugs with a narrow therapeutic index or those requiring rapid onset of action. Monitoring may be needed for certain concomitant medications.

Cost and Accessibility Considerations in the UK

One significant disadvantage of Mounjaro is its cost, which can be a barrier to access for many patients. In the UK, Mounjaro is available on the NHS, but prescribing is subject to strict eligibility criteria as outlined by NICE guidance. For type 2 diabetes, it may be considered as a treatment option within its licensed indication, typically after other therapies have been tried, in line with NICE guideline NG28 and local formulary decisions.

For weight management, NICE has recommended tirzepatide only within specialist weight management services and for a maximum of two years. Eligibility is restricted to adults with a BMI of at least 35 kg/m² (or at least 32.5 kg/m² in people from South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean family backgrounds) with at least one weight-related comorbidity, or adults with a BMI of at least 40 kg/m² (or at least 37.5 kg/m² in the aforementioned ethnic groups) without comorbidities.

The high cost of Mounjaro—often several hundred pounds per month when purchased privately—means that many patients rely on NHS provision. However, availability can be limited due to budget constraints, local formulary restrictions, and capacity within specialist services. Patients may face long waiting times for assessment or may not meet the specific criteria for NHS funding, leading to out-of-pocket expenses if they choose to pursue private treatment.

Additionally, there have been supply issues affecting GLP-1 receptor agonists in the UK, including Mounjaro, as documented by the Specialist Pharmacy Service and Department of Health and Social Care alerts. This can result in treatment interruptions or difficulty initiating therapy. Patients should discuss with their GP or specialist whether Mounjaro is realistically accessible and sustainable for their individual circumstances. The need for ongoing prescriptions and monitoring also requires regular healthcare appointments, which may be inconvenient or difficult to arrange. For those who do not meet NHS criteria or cannot afford private prescriptions, alternative evidence-based treatments for diabetes or weight management should be explored.

Who Should Avoid Taking Mounjaro?

Mounjaro is not suitable for everyone, and certain patient groups should avoid this medication due to safety concerns or lack of evidence. According to the UK Summary of Product Characteristics (SmPC), it is contraindicated in individuals with:

• Known hypersensitivity to tirzepatide or any excipients in the formulation

Pregnant women should not use Mounjaro. There is insufficient data on its safety during pregnancy, and animal studies have shown potential harm. Women of childbearing potential should use effective contraception during treatment, and Mounjaro should be discontinued at least one month before a planned pregnancy. For women using oral contraceptives, be aware that tirzepatide may reduce their effectiveness, particularly during the first 4 weeks after starting treatment and for 4 weeks after each dose increase. Consider using a non-oral contraceptive method or adding a barrier method during these periods.

Regarding breastfeeding, a benefit-risk assessment should be made, weighing the benefits of breastfeeding against the potential benefits of therapy, as it is not known whether tirzepatide is excreted in human milk.

Mounjaro is not licensed for use in children or adolescents under 18 years of age, as safety and efficacy have not been established in this population. It is also not indicated for type 1 diabetes or for the treatment of diabetic ketoacidosis.

Mounjaro is not recommended in patients with:

• Severe gastrointestinal disease, including severe gastroparesis, as Mounjaro further slows gastric emptying

Caution is advised in patients with:

• History of pancreatitis—careful consideration and monitoring are required

• Diabetic retinopathy—rapid glycaemic improvement may transiently worsen retinopathy, particularly in patients on insulin

No dose adjustment is required in renal impairment, though renal function should be monitored if significant gastrointestinal adverse effects or dehydration occur.

Patients should have a thorough discussion with their healthcare provider to assess whether Mounjaro is appropriate for their individual medical history, current health status, and treatment goals. Regular monitoring and follow-up are essential to identify and manage any adverse effects promptly.

If you experience any side effects, talk to your doctor, pharmacist or nurse. This includes any possible side effects not listed in the package leaflet. You can also report side effects directly via the Yellow Card Scheme at yellowcard.mhra.gov.uk.

Frequently Asked Questions