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
Mounjaro: The Slimming Injection Explained
Mounjaro (tirzepatide) is a prescription-only slimming injection licensed in the UK for weight management in adults. It's a once-weekly injectable medicine that works by activating two natural hormone pathways to help reduce appetite and support weight loss alongside lifestyle changes. Understanding how this weight loss injection works, who can access it, and what results to expect can help you decide if it's right for you. Our free BMI calculator above can help you check whether you might be eligible based on current NHS thresholds.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 as a Weight Loss Injection
Mounjaro is a dual GIP and GLP-1 receptor agonist — this means it activates two natural hormone pathways in your body simultaneously. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are hormones your gut normally produces after eating.
When you take Mounjaro, it:
- Signals your brain that you're full, reducing appetite and food cravings
- Helps you feel satisfied after smaller meals
- Slows how quickly food leaves your stomach
- Supports your body's ability to regulate blood sugar and fat use
This dual mechanism is what distinguishes Mounjaro from other weight loss injections like Wegovy (semaglutide), which activates only the GLP-1 pathway. In the SURMOUNT-1 clinical trial, participants using Mounjaro at the maximum 15 mg dose lost an average of approximately 20.9% of their body weight over 72 weeks — though individual results vary significantly.
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.
Mounjaro Eligibility, Doses, and Access in the UK
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 a weight-related health condition such as type 2 diabetes, high blood pressure, or cardiovascular disease. Lower BMI thresholds may apply if you're of South Asian, Chinese, Black African, or Black Caribbean heritage, as these groups face higher health risks at lower weights.
Mounjaro starts at 2.5 mg weekly and can be increased to a maximum of 15 mg, with at least four weeks between dose increases. Access in the UK is through either NHS specialist weight management services (via your GP) or private online prescribing following a clinical assessment. Try our free GLP-1 weight loss calculator to explore projected weight loss at different doses, and use our weight converter to track progress in stone, pounds, or kilograms.
- 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 available on the NHS in the UK?
Yes. NICE has approved tirzepatide (Mounjaro) for NHS use within specialist weight management services. However, access is through tier 3/4 specialist services, not routine GP prescription. You'll need a referral from your GP. Availability varies significantly by region, and waiting lists can be lengthy. Private access is also available through online prescribing.
What's the difference between Mounjaro and Wegovy as slimming injections?
Both are prescription-only weekly injections, but Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro starts at 2.5 mg and goes up to 15 mg; Wegovy starts at 0.25 mg and reaches 2.4 mg. Clinical trials showed similar average weight loss (~15–21%), though individual results vary.
What are the common side effects of Mounjaro as a slimming injection?
The most common side effects are nausea, diarrhoea, vomiting, and constipation, especially during dose increases. Most improve over time. Less common side effects include indigestion, fatigue, dizziness, injection site reactions, and hair loss. Serious side effects (severe stomach pain, gallbladder problems, serious allergic reactions) are rare but require immediate medical attention.
How quickly do you lose weight on Mounjaro?
Weight loss is typically gradual. Clinical trials showed average weight loss of approximately 15–21% of body weight over 72 weeks — roughly 0.5 kg per week or more. However, results vary considerably between individuals. Weight loss requires Mounjaro alongside a reduced-calorie diet and increased physical activity; the injection alone is not sufficient.
Can I get Mounjaro prescribed privately in the UK?
Yes. Mounjaro is available through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber. You don't need an NHS referral. If a prescriber assesses you as clinically suitable, a prescription will be issued. Costs vary by provider and dose, typically ranging from £130–£320 monthly.
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.