Can Mounjaro Cause Blood Clots? UK Evidence and Safety Guidance

Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist licensed in the UK for type 2 diabetes and weight management. As its use expands, patients naturally question potential side effects, including concerns about blood clots. Current clinical evidence does not establish a causal link between Mounjaro and increased thrombotic risk. The MHRA-approved Summary of Product Characteristics does not list thromboembolism as a known adverse reaction. This article examines the evidence surrounding Mounjaro and blood clot risk, clarifies relevant risk factors, and provides practical guidance for safe medication use under medical supervision.

Understanding Mounjaro and Blood Clot Concerns

Mounjaro (tirzepatide) is a relatively new medication licensed in the UK for the treatment of type 2 diabetes mellitus and, more recently, for weight management in adults with obesity or overweight with weight-related comorbidities (as per MHRA/EMC SmPC and NICE TA904). As a dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist, Mounjaro works by enhancing insulin secretion, suppressing glucagon release, slowing gastric emptying, and reducing appetite. These mechanisms contribute to improved glycaemic control and significant weight loss.

As with any medication, patients and healthcare professionals naturally have questions about potential adverse effects. Concerns about blood clots—medically termed venous thromboembolism (VTE), which includes deep vein thrombosis (DVT) and pulmonary embolism (PE)—have emerged in online discussions and patient forums. It is important to clarify that there is currently no established causal link between Mounjaro and an increased risk of blood clots based on available clinical trial data and post-marketing surveillance. The MHRA-approved Summary of Product Characteristics (SmPC) does not list thromboembolism as a known adverse reaction.

The confusion may arise from the fact that people with type 2 diabetes and obesity—the very populations prescribed Mounjaro—already have elevated baseline risks for cardiovascular and thrombotic events due to factors such as inflammation, endothelial dysfunction, and metabolic syndrome. Understanding the distinction between correlation and causation is essential when evaluating medication safety profiles.

This article examines the clinical evidence surrounding Mounjaro and thrombotic risk, identifies relevant risk factors and warning signs, and provides practical guidance for patients using this medication safely under medical supervision.

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Clinical Evidence on Mounjaro and Thrombotic Risk

The safety profile of Mounjaro has been extensively evaluated through clinical trial programmes, including the SURPASS trials (for type 2 diabetes) and the SURMOUNT trials (for weight management). These phase 3 studies compared tirzepatide against placebo and active comparators, monitoring participants for adverse events including cardiovascular outcomes. Importantly, blood clots were not identified as a significant adverse event in these comprehensive studies, and thrombotic events did not occur at rates higher than expected in the study populations, as confirmed in the European Medicines Agency's European Public Assessment Report (EPAR).

The Medicines and Healthcare products Regulatory Agency (MHRA) and the European Medicines Agency (EMA) have reviewed Mounjaro's safety data as part of the licensing process. The Summary of Product Characteristics (SmPC) for tirzepatide does not list thromboembolism or blood clots as known adverse reactions in section 4.8. The most commonly reported side effects remain gastrointestinal in nature, including nausea, vomiting, diarrhoea, and constipation, which are typical of GLP-1 receptor agonist medications.

Post-marketing pharmacovigilance continues to monitor real-world safety data as Mounjaro use expands. To date, regulatory authorities have not issued safety alerts regarding thrombotic risk. Dehydration from gastrointestinal side effects is a recognised concern with GLP-1 receptor agonists, and maintaining adequate hydration is important for all patients.

Patients should be reassured that current evidence does not support a direct mechanistic link between Mounjaro and blood clot formation. Ongoing surveillance and research will continue to refine our understanding of tirzepatide's long-term safety profile.

Risk Factors and Warning Signs to Monitor

Whilst Mounjaro itself does not appear to increase blood clot risk, patients prescribed this medication often have pre-existing risk factors for venous thromboembolism. Understanding these factors is crucial for appropriate risk stratification and monitoring. Key risk factors include:

• Obesity: Excess body weight increases venous stasis and inflammatory markers, elevating VTE risk two- to threefold

• Type 2 diabetes: Associated with hypercoagulability, endothelial dysfunction, and increased platelet reactivity

• Reduced mobility: Sedentary lifestyle or limited physical activity impairs venous return

• Previous VTE: Personal history of DVT or PE significantly increases recurrence risk

• Smoking: Damages blood vessel walls and promotes clot formation

• Hormonal factors: Oestrogen-containing contraceptives or hormone replacement therapy

• Recent surgery or prolonged immobilisation: Major operations or extended bed rest

• Active malignancy: Cancer increases thrombotic risk through multiple mechanisms

• Thrombophilia: Inherited or acquired clotting disorders

• Pregnancy and postpartum period: Hormonal changes and increased abdominal pressure

• Advancing age: Risk increases with age, particularly over 60 years

Warning signs of deep vein thrombosis include unilateral leg swelling, pain or tenderness (often in the calf), warmth, and redness. Pulmonary embolism symptoms may present as sudden breathlessness, sharp chest pain (worse with breathing), rapid heart rate, coughing up blood, or feeling faint (NHS DVT and PE guidance).

Patients starting Mounjaro should discuss their individual VTE risk profile with their healthcare provider. Those with multiple risk factors may benefit from additional preventive measures, such as maintaining adequate hydration (particularly important given Mounjaro's gastrointestinal effects) and staying physically active. For those at increased risk during long-haul travel, correctly fitted below-knee compression stockings may be beneficial, though these should be avoided in patients with peripheral arterial disease. Regular medication reviews ensure that any changes in risk status are appropriately addressed.

When to Seek Medical Advice

Patients taking Mounjaro should be aware of situations requiring prompt medical assessment, particularly regarding potential thrombotic complications. Immediate emergency care (999 or A&E attendance) is warranted if you experience:

• Sudden severe breathlessness or difficulty breathing

• Sharp chest pain, especially if worse when breathing deeply

• Coughing up blood or blood-stained sputum

• Sudden collapse or loss of consciousness

• Severe leg pain with significant swelling and colour change

These symptoms may indicate pulmonary embolism or extensive deep vein thrombosis, both requiring urgent investigation and treatment.

Contact your GP or NHS 111 within 24 hours if you notice:

• Persistent unilateral leg swelling that does not resolve with elevation

• New calf pain or tenderness, particularly if accompanied by warmth

• Unexplained breathlessness developing gradually

• Unusual fatigue or reduced exercise tolerance

In line with NICE guideline NG158 (Venous thromboembolic diseases), suspected DVT or PE requires same-day assessment, including clinical pre-test probability assessment (e.g., Wells score), D-dimer testing, and imaging (proximal leg ultrasound for suspected DVT or CTPA for suspected PE) as indicated. Interim anticoagulation may be considered when VTE is likely, pending definitive diagnosis.

It is equally important to maintain regular contact with your diabetes care team or weight management service whilst taking Mounjaro. Routine follow-up appointments allow monitoring of treatment response, assessment of side effects, and review of cardiovascular risk factors. NICE guidance (NG28) recommends regular HbA1c monitoring for diabetes patients and periodic cardiovascular risk assessment.

Patients should not discontinue Mounjaro without medical advice, even if concerned about potential side effects. Your prescriber can evaluate your individual risk-benefit profile and discuss alternative treatment options if appropriate. If you are planning surgery or will be immobilised for an extended period, inform your surgical team that you are taking Mounjaro, as temporary adjustments to your medication regimen or additional VTE prophylaxis may be considered based on your overall risk assessment.

Safe Use of Mounjaro: Guidance for Patients

Optimising safety whilst taking Mounjaro involves understanding proper medication use and adopting lifestyle measures that support overall cardiovascular health. Administration guidance includes:

• Inject subcutaneously once weekly, on the same day each week

• Rotate injection sites (abdomen, thigh, or upper arm) to prevent lipohypertrophy

• Store pens in the refrigerator (2–8°C); do not freeze; keep in outer carton to protect from light

• Follow dose escalation schedule as prescribed to minimise gastrointestinal side effects

• Note that Mounjaro pens are single-use only and should be disposed of after injection

Hydration is particularly important when taking Mounjaro, as gastrointestinal side effects such as vomiting and diarrhoea can lead to dehydration. Aim for adequate fluid intake throughout the day, particularly during the initial weeks of treatment when side effects are most common.

Lifestyle modifications that reduce thrombotic risk whilst supporting Mounjaro's therapeutic effects include:

• Regular physical activity: Aim for 150 minutes of moderate-intensity exercise weekly, as recommended by the UK Chief Medical Officers' Physical Activity Guidelines. Physical activity improves venous return and reduces VTE risk

• Smoking cessation: Access NHS stop smoking services for support

• Healthy diet: Follow a balanced eating plan that complements your weight management goals

• Medication adherence: Take all prescribed medications as directed. If you are already prescribed antiplatelet or anticoagulant therapy, continue as directed—do not start or stop these medications without medical advice

Before starting Mounjaro, provide your healthcare team with a complete medical history, including previous blood clots, cardiovascular disease, family history of thrombophilia, and all current medications and supplements. This information enables appropriate risk assessment and monitoring.

Patients should maintain realistic expectations about weight loss and understand that Mounjaro is most effective when combined with lifestyle interventions. Regular follow-up ensures that treatment remains appropriate and that any emerging concerns are addressed promptly. The current evidence base supports Mounjaro as a safe and effective treatment option when used appropriately under medical supervision.

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