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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What Are the Main Risks of Mounjaro in the UK?
Mounjaro (tirzepatide) is a prescription-only GLP-1 and GIP receptor agonist injection licensed for weight management in UK adults. Like all medicines, it carries potential risks and side effects you should understand before starting treatment. This guide covers NHS prescribing context, common and serious side effects, eligibility criteria, and what to discuss with your GP or prescriber. Understanding these risks helps you make an informed decision about whether Mounjaro 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.
Common Side Effects and Serious Risks to Know
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain — typically mild and improve over time as your body adjusts. Common side effects include indigestion, acid reflux, injection site reactions, fatigue, dizziness, and hair loss (alopecia).
Serious risks requiring immediate medical attention include severe stomach pain spreading to the back (possible pancreatitis), gallbladder problems, serious allergic reactions, and changes in vision. Mounjaro may also affect how oral medications are absorbed — always inform your prescriber of all medications you take. For those with a history of thyroid cancer or multiple endocrine neoplasia (MEN2), Mounjaro is not suitable.
- Report severe or persistent side effects to your GP immediately
- Do not stop Mounjaro without medical guidance
- Nausea typically reduces as your body adjusts to each dose
Clinically BackedWeight Loss
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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.
NHS Prescribing, Eligibility, and Private Options
In the UK, Mounjaro is prescribed through NHS tier 3 specialist weight management services following NICE approval. You'll need referral from your GP and assessment by a specialist — waiting lists vary significantly by region. Alternatively, private UK-licensed prescribers offer online clinical assessments; if suitable, a prescription is issued and dispensed by GPhC-registered pharmacies. Both routes require you to meet eligibility criteria: BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions. Use our free BMI calculator to check your starting point, and discuss risks and benefits with your healthcare provider before commencing tirzepatide treatment.
Mounjaro vs Wegovy: Risk ComparisonBoth Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK with broadly similar side effect profiles. Mounjaro uses dual GIP and GLP-1 mechanisms versus Wegovy's single GLP-1 pathway — clinical trials suggest Mounjaro may produce slightly higher average weight loss, though individual results vary considerably. Side effect risks are comparable; both require dose titration over weeks. The main practical differences are injection frequency (Mounjaro weekly, Wegovy weekly), maximum doses, and trial data context. Your prescriber will assess which is more appropriate based on your health profile, medical history, and personal 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 should I do if I experience severe stomach pain on Mounjaro?
Seek urgent medical attention immediately. Severe stomach pain spreading to the back may indicate pancreatitis, a serious condition requiring emergency assessment. Contact your GP or attend A&E. Do not assume it will pass. Always report severe or worsening symptoms to your healthcare provider.
Can I take Mounjaro if I'm on the contraceptive pill?
Mounjaro may reduce absorption of oral medications, including contraceptive pills. If you take oral contraceptives, add a barrier method or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. Discuss this with your prescriber.
How long do Mounjaro side effects typically last?
Most common side effects (nausea, diarrhoea, stomach upset) tend to improve within days to weeks as your body adjusts to each dose. If side effects persist or worsen, contact your GP. Some people experience hair loss, which may continue; this should be monitored by your healthcare provider.
Am I eligible for Mounjaro through the NHS?
NHS eligibility requires BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions (diabetes, high blood pressure, cardiovascular disease, sleep apnoea). You'll need referral to a tier 3 specialist weight management service. Check your BMI using our free BMI calculator, then discuss eligibility with your GP.
What's the difference between Mounjaro and Ozempic for weight loss?
Mounjaro (tirzepatide) is licensed for weight management at doses up to 15mg. Ozempic (semaglutide) is licensed for type 2 diabetes management only — using it for weight loss is off-label. Wegovy (also semaglutide) is the licensed weight management version. Ask your prescriber which is appropriate for your situation.
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.