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
Free tracker included with every download this week
What Is Mounjaro? A UK Health Guide
Mounjaro is a prescription-only weight management injection licensed in the UK for adults with obesity or overweight with weight-related health conditions. It contains tirzepatide, a dual GIP and GLP-1 receptor agonist that works differently from other weight loss injections. Mounjaro may help reduce appetite, support feeling fuller after meals, and assist with sustainable weight loss when combined with diet and physical activity changes. This guide explains how Mounjaro works, who may be eligible, what to expect, and how to access it through the NHS or privately in the UK.
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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 What Makes It Different
Mounjaro activates two natural hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) — which is what sets it apart from single-action weight loss injections like Wegovy (semaglutide). This dual mechanism may help regulate appetite more effectively, reduce food cravings, and support feeling fuller after smaller meals. The injection also slows how quickly food leaves the stomach, though this effect typically decreases over time as your body adjusts.
Mounjaro is not insulin and does not work the same way insulin does. It's a once-weekly injection administered at home using a pre-filled KwikPen device. Each pen contains 4 doses (4 weeks' supply) and must be stored in a refrigerator. Clinical trials (SURMOUNT-1) showed that adults on Mounjaro lost an average of 15–21% of their body weight over 72 weeks — though individual results vary significantly and depend on dose, diet, and lifestyle changes.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
UK Eligibility, Access, and What to Expect
You may be eligible for Mounjaro in the UK if you have a BMI of 30 or above, or a BMI of 27–29.9 with at least one weight-related health condition such as type 2 diabetes, prediabetes, high blood pressure, or cardiovascular disease. Lower BMI thresholds apply for adults of South Asian, Chinese, Black African, or Black Caribbean heritage due to higher health risks at lower weights. Use our free BMI calculator above to check your personal BMI and determine potential eligibility. Mounjaro is available through NHS specialist weight management services (referral from your GP required) or through private online prescribing with a UK-licensed prescriber. Side effects are common initially — nausea, diarrhoea, and constipation are most frequent — but often improve as your body adjusts to each dose increase.
- 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.
Is Mounjaro the same as Wegovy?
No. Both are prescription weight management injections, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1-only agonist. Clinical trials showed slightly higher average weight loss with Mounjaro, but individual results vary. Both require clinical assessment and are prescription-only.
How long does Mounjaro take to work?
Weight loss typically begins within the first 4 weeks, though the most significant results occur over 12–20 weeks as you increase through doses. Most people reach their target dose (15 mg) by week 21. Results depend on diet, physical activity, and individual metabolism — MyWeightCheck's GLP-1 calculator can help estimate potential outcomes.
Can I get Mounjaro on the NHS in the UK?
Yes, but availability varies by region. Mounjaro is approved by NICE for NHS specialist weight management services. Access requires a GP referral to a tier 3 or 4 specialist service. Waiting lists can be long in many areas. Private access is available through online prescribing without NHS referral.
What are the most common Mounjaro side effects?
Nausea, diarrhoea, vomiting, and constipation are very common, especially at the start or when increasing doses. Most people find these improve within 1–2 weeks. Less common side effects include fatigue, dizziness, headache, and injection site reactions. Seek urgent medical attention for severe stomach pain or allergic reactions.
Does Mounjaro work without diet and exercise changes?
No. Mounjaro is most effective alongside a reduced-calorie diet and increased physical activity. It helps manage appetite and reduce cravings, but sustainable weight loss requires lifestyle changes. Track your progress with our free progress tracker to monitor improvements in both weight and healthy habits.
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.