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 Warnings vs Wegovy — What You Need to Know
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work differently and have different safety profiles. Understanding the warnings and potential risks of each medication helps you make an informed decision with your prescriber. This guide compares the key safety considerations, clinical trial data, and eligibility criteria for both treatments so you can have an evidence-based conversation with your GP or licensed prescriber about which might be suitable 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.
Key Differences in Mechanism and Clinical Trial Safety Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single pathway. This difference in mechanism affects how they work and their side effect profiles.
In the SURMOUNT-1 clinical trial, Mounjaro at 15 mg achieved average weight loss of approximately 20.9% body weight. In the STEP-1 trial, Wegovy at 2.4 mg achieved approximately clinically studied weight loss. Both trials included thousands of participants and tracked safety carefully. The most commonly reported side effects in both trials were gastrointestinal: nausea, diarrhoea, vomiting, and constipation. However, individual responses vary significantly, and not everyone experiences the same side effects at the same severity.
Both medications carry warnings about potential pancreatitis (inflammation of the pancreas), gallbladder problems, and changes in vision. Serious allergic reactions are rare but possible with any medication. Your prescriber will assess your personal risk factors before recommending either treatment.
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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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Choosing Between Them — A Prescriber Decision
Neither Mounjaro nor Wegovy is inherently "safer" than the other—suitability depends entirely on your individual health profile, medical history, and response to treatment. Your GP or licensed prescriber will consider factors including: your current BMI and weight-related health conditions, any previous medication reactions, kidney or pancreas function, gallbladder status, and whether you're taking other medications that might interact.
Both require a UK prescription and clinical assessment before dispensing. Try our free GLP-1 weight loss calculator to understand potential outcomes at different doses, and use our BMI calculator to determine your eligibility starting point. Your prescriber will discuss the specific warnings, your personal risk level, and which medication aligns best with your health needs and circumstances.
- 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.
What are the main warnings for Mounjaro in the UK?
UK warnings include risk of pancreatitis (seek urgent care for severe stomach pain), gallbladder problems, thyroid tumour risk (avoid if personal/family history of thyroid cancer), diabetic retinopathy complications, and allergic reactions. Mounjaro may reduce effectiveness of oral contraceptives—use barrier contraception for 4 weeks after starting and after each dose increase. Not suitable in pregnancy or if breastfeeding.
What are the main warnings for Wegovy in the UK?
Wegovy warnings include pancreatitis risk, gallbladder disease, thyroid tumour risk, diabetic retinopathy worsening, and serious allergic reactions. Like Mounjaro, it may affect oral contraceptive absorption. Not for use in pregnancy or while breastfeeding. Wegovy is not recommended if you have a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2.
Which has fewer side effects, Mounjaro or Wegovy?
Both cause similar gastrointestinal side effects (nausea, diarrhoea, constipation, vomiting) at similar frequencies in clinical trials. Mounjaro may have slightly different side effect patterns due to its dual mechanism, but individual experience varies greatly. Some people tolerate one better than the other—this is unpredictable and assessed during treatment.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, switching is possible, but only under prescriber supervision. There is no official conversion chart—your prescriber will assess the appropriate dose and monitor your response. You cannot simply switch between them without medical guidance. Any switch requires a new clinical assessment.
Are both available on the NHS in the UK?
Both Mounjaro and Wegovy are NICE-approved and available through NHS specialist tier 3 weight management services, though regional availability varies significantly and waiting lists can be lengthy. Both are also available privately through registered online prescribing services following clinical assessment. Availability depends on your local NHS integrated care board.
Ready to take control of your weight loss journey?
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