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, Thyroid Cancer, and What the Evidence Shows
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) carry a theoretical thyroid cancer risk based on animal studies, which is why the MHRA requires additional monitoring. This guide explains what the clinical evidence actually shows, how the two medications compare on safety grounds, and what you should discuss with your prescriber before starting either treatment. Individual safety assessments are always required.
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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: Thyroid Safety and Monitoring
Animal study findings: In preclinical studies, both tirzepatide (Mounjaro) and semaglutide (Wegovy) were associated with thyroid C-cell tumours in rodents. This is why both medications carry a Black Triangle (▼) status in the UK, requiring additional monitoring by the MHRA. However, animal studies do not always translate to human risk.
Human clinical trial data: The SURMOUNT trials (Mounjaro, up to 72 weeks) and STEP trials (Wegovy, up to 68 weeks) did not show increased thyroid cancer incidence in participants. No cases of medullary thyroid carcinoma (MTC) were reported in either trial series. Thyroid-related adverse events were reported but at similar rates between treatment and placebo groups. Ongoing real-world monitoring continues post-approval.
Current status: Neither medication is contraindicated due to thyroid cancer risk, but both require clinical assessment and informed consent before prescription. Your prescriber will discuss this risk with you.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Mounjaro vs Wegovy: Thyroid Safety Comparison
Both medications have the same theoretical thyroid risk origin — animal model findings — so neither is categorically safer than the other on this specific grounds. The choice between Mounjaro and Wegovy depends on other factors: Mounjaro uses a dual GIP+GLP-1 mechanism and may support greater average weight loss (15–21% body weight vs approximately 15% with Wegovy), but both are prescription-only and require individual clinical assessment. Your GP or prescriber will weigh thyroid risk alongside your personal health history, family background, and weight management needs. Use our free GLP-1 weight loss calculator to explore projected outcomes at different doses, then discuss both medication options with your prescriber.
- 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 safe if there's thyroid cancer in my family?
A family history of thyroid cancer or medullary thyroid carcinoma (MTC) is an important factor. You must disclose this to your prescriber before starting Mounjaro or Wegovy. Both medications may not be suitable if you have a personal or strong family history of MTC or multiple endocrine neoplasia type 2 (MEN2). Your prescriber will assess individual risk carefully.
Did any patients in the Mounjaro trials develop thyroid cancer?
No cases of medullary thyroid carcinoma were reported in the SURMOUNT clinical trials of Mounjaro (up to 72 weeks). Thyroid-related side effects occurred at similar rates in treatment and placebo groups. However, the trials were not designed to prove thyroid safety — they were weight loss efficacy trials — so ongoing monitoring continues post-approval.
Why do both Mounjaro and Wegovy have Black Triangle status?
Black Triangle (▼) status means the MHRA requires additional monitoring because of findings from animal studies showing thyroid C-cell tumours in rodents at high doses. This does not mean the medications are unsafe in humans, but it signals that real-world safety data is being actively collected and reviewed.
Can I take Mounjaro if I have a history of thyroid problems?
If you have thyroid disease (such as hypothyroidism), this doesn't automatically exclude you from Mounjaro. However, you must inform your prescriber of your full thyroid history. Thyroid function may change during weight loss, so your thyroid medication dose might need adjustment. Your GP can monitor this.
What should I ask my prescriber about thyroid cancer risk?
Ask whether Mounjaro or Wegovy is suitable given your personal and family health history, whether thyroid monitoring is recommended during treatment, what symptoms to watch for (neck lumps, difficulty swallowing, persistent hoarseness), and how often your thyroid function will be checked. Use our BMI calculator to assess your current weight status, then have a full informed discussion with your prescriber.
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