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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How Mounjaro Works for Weight Loss
Mounjaro (tirzepatide) is a prescription-only weight management injection that may help support weight loss when combined with a reduced-calorie diet and increased physical activity. It's licensed in the UK as a once-weekly injection that works by activating two natural hormone pathways — GIP and GLP-1 — to help regulate appetite and reduce food cravings. Clinical trials showed participants lost an average of 15–21% of their body weight, though individual results vary significantly. This guide explains how Mounjaro works, UK eligibility criteria, doses, and how to access it through the NHS or private prescribing.
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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 Mounjaro Works and Weight Loss Mechanism
Mounjaro's dual mechanism makes it different from other weight management UK options. It activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptors — natural hormones your gut produces after eating.
- Appetite regulation: Both pathways signal to your brain that you feel fuller, potentially reducing food intake and cravings.
- Gastric emptying: Mounjaro slows how quickly food leaves your stomach, which may help you feel satisfied longer after meals (though this effect often reduces over time).
- Fat utilisation: The medication helps your body manage how it uses fat for energy.
In the SURMOUNT-1 clinical trial, participants on the 15 mg dose lost approximately 20.9% of their body weight over 72 weeks — substantially more than GLP-1-only medications like Wegovy. However, these results were achieved alongside structured dietary changes and physical activity, not the medication alone.
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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.
Mounjaro Eligibility and NHS Access in the UK
In the UK, you may be eligible for prescription weight loss treatment with Mounjaro if you have a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. For people of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower thresholds due to higher cardiovascular risk at lower BMI levels.
NHS access: Available through specialist tier 3 weight management services — you'll need a GP referral. Waiting lists vary significantly by region. Private access: Available through online prescribing following a clinical assessment, typically costing £129–£320 monthly depending on dose. Use our free BMI calculator above to check your eligibility range, and our GLP-1 weight loss calculator to project potential results 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.
How much weight can you lose with Mounjaro?
Clinical trials (SURMOUNT-1) showed average weight loss of 15–21% of body weight at the maximum 15 mg dose over 72 weeks, alongside diet and exercise. Results vary widely between individuals — some lose more, some less. It's not clinically studied weight loss, and individual outcomes depend on adherence, lifestyle changes, and personal factors. Your prescriber can discuss realistic expectations for your situation.
Is Mounjaro available on the NHS in the UK?
Yes, Mounjaro (tirzepatide) has been approved by NICE for use within NHS specialist weight management services. However, access is through tier 3 services, typically via GP referral, and availability varies significantly by region. Waiting lists can be very long in many areas. Private prescribing is also available without NHS referral.
What's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist; Wegovy (semaglutide) is a GLP-1-only agonist. Clinical trials suggest Mounjaro may produce slightly greater average weight loss (15–21% vs approximately 15% for Wegovy), but both are effective. The best option depends on individual health, side effect tolerance, and prescriber recommendation. Both are prescription-only.
What are common side effects of Mounjaro for weight loss?
Very common side effects include nausea, diarrhoea, vomiting, and stomach pain — most mild and reducing over time. Common effects include indigestion, injection site reactions, fatigue, and dizziness. Serious but rare side effects include severe pancreatitis symptoms (severe stomach pain radiating to the back) and gallbladder problems. Report any concerning symptoms to your prescriber immediately.
How do you use Mounjaro for weight loss — injection frequency and doses?
Mounjaro is a once-weekly subcutaneous injection (stomach, thigh, or upper arm). Starting dose is 2.5 mg weekly, increasing by 2.5 mg every four weeks up to the maximum 15 mg. Each KwikPen contains four doses (four weeks' supply). Doses are adjusted based on tolerance and response. Your prescriber will guide your dose escalation schedule.
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.