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 and Thyroid Cancer — What UK Patients Should Know
Mounjaro (tirzepatide) carries a boxed warning about thyroid cancer risk based on animal studies, though human cases remain extremely rare. This guide explains what the warning means, how it affects UK eligibility, and what to discuss with your GP or prescriber before starting treatment. Understanding the evidence helps you make an informed decision about whether this prescription weight loss medication is right for you.
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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.
Thyroid Cancer Warning and What It Means
Mounjaro carries a boxed warning (Black Triangle ▼ status) from the MHRA because animal studies in rodents showed an increased risk of thyroid C-cell tumours at doses higher than those used in humans. However, this warning does not mean the medication will cause thyroid cancer in people — it means the risk cannot yet be ruled out based on available evidence.
- No cases of thyroid cancer have been confirmed in human clinical trials of tirzepatide (Mounjaro)
- The warning applies specifically to people with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN2)
- For most people without this history, the actual risk is considered very low in clinical practice
- The MHRA continues to monitor safety data as more people use the medication
Your prescriber will assess your individual thyroid cancer risk during clinical assessment before issuing a prescription.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Who Should Not Take Mounjaro Due to Thyroid Cancer Risk
Mounjaro is contraindicated (not suitable) if you have:
- Personal history of medullary thyroid carcinoma (MTC)
- Multiple Endocrine Neoplasia syndrome type 2 (MEN2)
- Family history of medullary thyroid carcinoma or MEN2
If you have any thyroid concerns, nodules, or a family history of thyroid disease, tell your GP or online prescriber before starting Mounjaro. A prescriber can assess whether this medication is safe for you and discuss alternative weight loss options if needed. You can calculate your BMI using our free BMI calculator to understand your starting point and discuss weight management options 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.
Does Mounjaro actually cause thyroid cancer in people?
No confirmed cases of thyroid cancer have occurred in human clinical trials of Mounjaro. The warning is based on animal studies only. However, the MHRA requires monitoring because thyroid C-cell tumours were seen in rodents at high doses, and long-term human data is still being collected.
Should I be worried about the thyroid cancer warning?
If you have no personal or family history of medullary thyroid carcinoma (MTC) or MEN2 syndrome, the actual risk is considered very low. Most people taking Mounjaro will not develop thyroid problems. However, it's important to discuss your individual risk with your GP or prescriber before starting.
What is medullary thyroid carcinoma and why does it matter for Mounjaro?
Medullary thyroid carcinoma (MTC) is a rare type of thyroid cancer. GLP-1 and GIP medications like Mounjaro can stimulate C-cells in the thyroid, which is why people with MTC or a family history of it cannot take these medications. MTC runs in families in some cases (MEN2 syndrome), so family history is important to disclose.
What happens if I take Mounjaro and develop thyroid symptoms?
Stop taking Mounjaro and contact your GP immediately if you develop a lump in your neck, persistent hoarseness, difficulty swallowing, or shortness of breath. These could indicate thyroid problems, though they are extremely rare. Any concerns about thyroid health should be discussed with your doctor.
Can I take Mounjaro if I have other thyroid conditions like hypothyroidism?
If you have hypothyroidism or take thyroid medication, you can usually take Mounjaro, but your prescriber must assess this. Inform your GP or online prescriber of all thyroid conditions and medications. Your thyroid function may need monitoring. Discuss your full medical history before starting any weight loss medication.
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.