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 Long-Term Side Effects and Cancer Risk in the UK
Mounjaro (tirzepatide) is a relatively new weight management injection approved for UK use, so long-term safety data is still being gathered. While clinical trials and early monitoring have not identified a specific cancer risk, tirzepatide carries a black triangle (▼) status from the MHRA, meaning it requires additional safety monitoring. This guide explains what the current evidence shows about long-term side effects, cancer risk, and how the NHS monitors safety for patients prescribed Mounjaro in specialist weight management services.
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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 the Evidence Currently Shows About Long-Term Safety
Mounjaro was approved for weight management in the UK in 2023, which means long-term follow-up data beyond 2–3 years is limited. The main clinical trial data comes from the SURMOUNT programme, which tracked participants for approximately 72 weeks (1.5 years). Early safety monitoring has not revealed an increased cancer risk at this timeframe.
The MHRA assigned black triangle (▼) status to tirzepatide, a standard measure for newer medicines requiring enhanced safety surveillance. This does not mean the drug causes cancer — rather, it means the MHRA actively monitors reported side effects and any emerging safety signals. All patients prescribed Mounjaro in the NHS are part of this ongoing safety monitoring system.
GLP-1 and GIP receptor agonists work by activating natural hormone pathways, not by introducing foreign chemicals. Animal studies have not identified cancer risk, though these do not always directly predict human outcomes. Ongoing real-world data from NHS prescribing will continue to build the safety evidence as more people use tirzepatide long-term.
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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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Reported Long-Term Side Effects and MHRA Monitoring
Reported long-term side effects of Mounjaro include gastrointestinal issues (nausea, diarrhoea, constipation), injection site reactions, fatigue, and hair loss. Some patients report that GI symptoms improve over time as the body adjusts, whilst others experience persistent effects that may affect quality of life.
The MHRA continuously reviews adverse event reports. If any new safety signal emerges — including cancer risk — the regulator is required to act. The NHS tier 3 weight management services where Mounjaro is prescribed also monitor patient outcomes and can escalate concerns. Your GP or prescriber can discuss specific long-term risks based on your individual health profile and family history. If you experience unusual symptoms or changes whilst taking Mounjaro, report them to your healthcare team immediately.
Patients prescribed Mounjaro via the NHS are monitored through specialist weight management services, which track weight loss progress, blood pressure, blood sugar control, and any adverse effects. If you're considering Mounjaro, calculating your current BMI using our free BMI calculator can help you understand your eligibility and the potential health benefits of weight loss, which often outweigh the monitored risks of the medication itself.
- 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.
Does Mounjaro cause cancer?
Current evidence does not show that Mounjaro (tirzepatide) causes cancer. Clinical trials over 72 weeks found no increased cancer risk. The MHRA has assigned black triangle status for enhanced monitoring, not because cancer is suspected, but as a precaution with newer medicines. Long-term data beyond 3 years is still being gathered. Any emerging safety signal would prompt MHRA action.
Why does Mounjaro have black triangle status?
Black triangle (▼) status means the MHRA requires additional safety surveillance for new or recently approved medicines. It's a standard regulatory measure, not evidence of a safety problem. Tirzepatide received this status because it's relatively new. The MHRA monitors all reported side effects and can modify guidance if safety concerns emerge.
What long-term side effects are most common with Mounjaro?
Reported long-term side effects include gastrointestinal symptoms (nausea, diarrhoea, constipation), injection site reactions, fatigue, and hair loss. Most GI effects are most severe during dose escalation and may improve with time. Persistent side effects affecting quality of life should be discussed with your prescriber.
How does the NHS monitor Mounjaro safety?
The NHS monitors Mounjaro through tier 3 specialist weight management services. Patients receive regular check-ups assessing weight loss, blood pressure, blood sugar, kidney function, and any adverse effects. Healthcare teams report serious side effects to the MHRA, contributing to ongoing safety surveillance.
Is Mounjaro safer than other weight loss injections like Wegovy?
Both Mounjaro and Wegovy are prescription-only medications approved for UK use with ongoing safety monitoring. Mounjaro uses dual GIP and GLP-1 mechanisms, whilst Wegovy uses GLP-1 alone. Both carry similar common side effects. Individual safety depends on your health profile — your prescriber will assess which is appropriate for you.
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