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
Free tracker included with every download this week
Mounjaro and Breast Cancer — What You Need to Know
Breast cancer risk is a legitimate health concern when considering any weight management medication. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections used in the UK, and both have been assessed for safety including cancer risk by UK regulators. This guide compares what the clinical evidence shows about these two medications and breast cancer, helping you understand the facts before discussing options with your GP.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
Clinical Trial Data and Breast Cancer Risk
The key difference between Mounjaro and Wegovy lies in their mechanisms and the clinical trial data available:
- Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. The SURMOUNT trials, which assessed safety and efficacy over 72 weeks, did not show an increased breast cancer risk compared to placebo. However, post-marketing surveillance continues.
- Wegovy (semaglutide) is a GLP-1 receptor agonist only. The STEP trials similarly found no increased breast cancer risk in the trial period. Long-term safety data continues to be collected.
Both medications have been approved by the MHRA and assessed by NICE for weight management in UK adults. Neither trial was specifically designed to detect rare cancer risks, which is why ongoing monitoring and real-world evidence remain important. Your GP can discuss the most current safety data with you.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Individual Suitability and Clinical Assessment
Whether Mounjaro or Wegovy is more suitable depends on your individual health profile, medical history, and personal risk factors — not on breast cancer risk alone. A UK-licensed prescriber will assess:
- Your personal and family history of cancer
- Existing health conditions and medications
- Your BMI and eligibility criteria (typically BMI ≥30, or BMI 27–29.9 with weight-related health conditions)
- Your response to previous weight loss treatments
Both medications are prescription-only and require clinical assessment before being prescribed. Use our free GLP-1 weight loss calculator and BMI calculator to understand your starting point, then discuss your options with your GP, who can weigh all factors specific to you.
- 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.
Is Mounjaro safer than Wegovy regarding breast cancer risk?
Clinical trials for both medications did not show increased breast cancer risk. However, neither trial was designed specifically to detect rare cancer risks. Both have MHRA approval and NICE assessment. Individual safety profiles depend on your personal health factors, which your prescriber will assess. No medication is definitively "safer" — suitability is personal.
Should I avoid GLP-1 medications if I have a family history of breast cancer?
Not necessarily. Family history is an important factor your prescriber must know about, but it doesn't automatically exclude you from GLP-1 treatment. Your GP or prescriber will assess your individual risk profile, discuss the evidence, and help you make an informed decision. Do mention your family history during your clinical assessment.
What is the difference between Mounjaro's tirzepatide and Wegovy's semaglutide?
Mounjaro contains tirzepatide, a dual GIP and GLP-1 receptor agonist. Wegovy contains semaglutide, a GLP-1 receptor agonist only. The dual mechanism in Mounjaro may lead to slightly different effects, but both work on appetite and blood sugar regulation. Clinical trials showed both support weight loss alongside diet and exercise.
How long do I need to take Mounjaro or Wegovy?
This depends on your individual response and health goals. Some people continue long-term; others use them as part of a structured weight loss programme. Your prescriber will review this with you regularly. Stopping either medication should be discussed with your healthcare provider, as weight regain can occur.
Where can I get Mounjaro or Wegovy prescribed in the UK?
Through NHS specialist tier 3 weight management services (with a GP referral, though waiting lists vary by region) or through private online prescribing services following a clinical assessment. Both require a prescription from a UK-licensed prescriber. Neither is available over the counter or without clinical evaluation.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.