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 Weight Loss Injection — How It Works in the UK
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection licensed in the UK for adults with obesity or overweight with health conditions. It works by activating two natural hormone pathways to help regulate appetite and support feeling fuller after meals. Clinical trials showed that Mounjaro may help participants lose an average of 15–21% of their body weight, though individual results vary significantly. Unlike some other weight loss injections, Mounjaro uses a dual mechanism, which may make it effective for weight management alongside diet and physical activity changes.
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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.
How Does Mounjaro Work for Weight Loss?
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in your body. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are natural hormones that help regulate appetite, blood sugar, and how your body uses fat.
When you use Mounjaro, these activated pathways help:
- Reduce hunger signals sent to your brain
- Support feeling fuller after eating smaller portions
- Slow down how quickly food leaves your stomach
- Improve how your body processes glucose and utilises fat
Mounjaro is delivered as a weekly KwikPen injection, which you administer yourself at home. The dose starts at 2.5 mg and is gradually increased over several weeks to a maximum of 15 mg, depending on how you respond and your prescriber's recommendations. This gradual increase, called dose titration, helps your body adjust and typically reduces side effects.
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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
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Who Can Get Mounjaro Weight Loss Injections in the UK?
To be eligible for Mounjaro in the UK, you must generally have a BMI of 30 or above (obesity), or a BMI of 27–29.9 (overweight) if you have at least one weight-related health condition such as type 2 diabetes, prediabetes, high blood pressure, or cardiovascular disease. People of South Asian, Chinese, Black African, or Black Caribbean heritage may qualify at lower BMI thresholds due to higher health risks at these levels.
Mounjaro is prescription-only and requires a clinical assessment. In the UK, you can access Mounjaro through NHS specialist weight management services (following a GP referral) or through private online prescribing with a UK-licensed prescriber. You cannot purchase it without a valid prescription. Use our free BMI calculator above to check your starting point, and our GLP-1 weight loss calculator to see potential outcomes at different doses.
- 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 is the difference between Mounjaro and Wegovy for weight loss?
Both Mounjaro and Wegovy are prescription-only weekly injections, but Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy contains semaglutide (GLP-1 only). Clinical trials showed Mounjaro may achieve slightly higher average weight loss (15–21% vs ~15% for Wegovy), but individual results vary. Your prescriber will assess which is more suitable for you based on your health profile and medical history.
How much weight can I lose with Mounjaro?
In the SURMOUNT-1 clinical trial, participants lost an average of 15–21% of their body weight over 72 weeks at the maximum 15 mg dose, alongside diet and exercise changes. However, weight loss varies significantly between individuals. Use our GLP-1 weight loss calculator on this page to estimate potential results at different doses based on your current weight.
Is Mounjaro available on the NHS in the UK?
Yes, NICE has approved tirzepatide (Mounjaro) for use within NHS specialist tier 3 weight management services. However, NHS access requires a GP referral to your local specialist service and availability varies by region. Waiting lists can be several months in some areas. Private access is also available through UK-licensed online prescribers following a clinical assessment.
What are the most common side effects of Mounjaro?
The most common side effects are nausea, diarrhoea, vomiting, and constipation, particularly when starting or increasing doses. These usually improve over time as your body adjusts. Other side effects may include stomach pain, indigestion, fatigue, and dizziness. Serious side effects are rare but include severe stomach pain (possible pancreatitis) — seek immediate medical attention if this occurs.
Can I use Mounjaro if I'm taking other medications?
Mounjaro may affect how your body absorbs some oral medications, particularly oral contraceptives — if you use the pill, your prescriber may recommend adding a barrier contraceptive method for 4 weeks after starting and after each dose increase. If you take diabetes medications, doses may need adjusting. Always inform your prescriber of all medications you're currently taking before starting Mounjaro.
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.