The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Mounjaro Side Effects and Cancer Risk — What UK Patients Need to Know
If you're considering Mounjaro (tirzepatide) for weight management in the UK, understanding potential side effects—including cancer risk—is essential. This guide explains what the evidence currently shows about Mounjaro safety, draws on NHS guidance, and clarifies what prescribers assess before offering this prescription-only medication. Cancer risk is a key concern for many patients, so we address this directly and honestly, based on clinical trial data and regulatory information from the MHRA.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
What Does the Evidence Show About Mounjaro and Cancer Risk?
Clinical trials for Mounjaro (tirzepatide) did not show a significantly elevated cancer risk compared to placebo. However, the MHRA continues to monitor the medication under Black Triangle (▼) status, meaning extra safety information is being collected. The SURMOUNT trials, which studied tirzepatide over 72 weeks, reported cancer as an uncommon adverse event—but not at rates that raised specific regulatory concerns.
The medication works as a dual GIP and GLP-1 receptor agonist. Some animal studies have suggested potential thyroid effects in rodents at very high doses, but human clinical evidence does not replicate these findings. A prescriber will assess your individual health history, including any personal or family history of thyroid cancer, before offering Mounjaro. If you have a history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia (MEN) syndrome type 2, you will not be eligible for this medication.
- Black Triangle status means ongoing MHRA monitoring—not proof of increased risk
- No significant cancer signal emerged in SURMOUNT clinical trials
- Thyroid history is assessed during clinical evaluation
- Report any new health concerns to your prescriber immediately
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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Other Common Mounjaro Side Effects and When to Seek Help
While cancer risk is a specific concern many patients ask about, the most frequently reported side effects of Mounjaro are gastrointestinal: nausea, diarrhoea, vomiting, and constipation. These tend to improve as your body adjusts, particularly if you increase the dose slowly as recommended. Injection site reactions, fatigue, and hair loss are also reported.
Seek urgent medical attention if you experience: severe, persistent stomach pain (possible pancreatitis), serious allergic reactions (swelling of face or throat), changes in vision, or gallbladder symptoms (pain in upper abdomen). Your GP or prescriber should be notified of any unusual symptoms. Use our free BMI calculator and GLP-1 weight loss calculator to understand your eligibility and expected outcomes before discussing this medication with your healthcare provider.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Can Mounjaro cause cancer in the UK?
Clinical trials did not show a significantly elevated cancer risk. The MHRA monitors tirzepatide under Black Triangle status for safety, but this means extra vigilance, not proven harm. Your prescriber will assess your health history, including thyroid concerns, before offering the medication. Always report new symptoms to your GP.
Why does Mounjaro have Black Triangle status?
Black Triangle (▼) status indicates the MHRA is collecting extra safety information on the medication. This is a precautionary measure, not confirmation of serious risk. It means healthcare professionals and patients should report any adverse events promptly to support ongoing safety monitoring.
Who cannot take Mounjaro due to cancer or thyroid risk?
Mounjaro is contraindicated (not suitable) if you have a history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia (MEN) syndrome type 2. If you have any personal or family history of thyroid cancer, inform your prescriber—they will assess whether Mounjaro is appropriate for you.
What are the most common Mounjaro side effects in the UK?
The most common side effects are gastrointestinal: nausea, diarrhoea, vomiting, and constipation. These often improve as your body adjusts. Injection site reactions, fatigue, and hair loss are also reported. Most side effects are mild to moderate and manageable.
How do I get Mounjaro prescribed in the UK?
Mounjaro is prescription-only and available through NHS specialist tier 3 weight management services (via GP referral) or private online prescribing following a clinical assessment. A UK-licensed prescriber will review your health history, including any cancer or thyroid concerns, before deciding if it's suitable for you.
Ready to take control of your weight loss journey?
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