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 and How Does It Work?
Mounjaro (tirzepatide) is a prescription-only weight loss injection licensed in the UK for adults living with excess weight. It's a dual GIP receptor and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to help regulate appetite and support feeling fuller after meals. Unlike some older weight loss medications, Mounjaro works by mimicking hormones your gut naturally produces when you eat. In clinical trials, it helped participants lose an average of 15–21% of their body weight when combined with a reduced-calorie diet and increased physical activity. Individual results vary significantly, and it's available only through prescription following a clinical assessment by your GP or a licensed prescriber.
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 and Clinical Trial Results
Mounjaro works by activating two hormone pathways simultaneously. The GIP (glucose-dependent insulinotropic polypeptide) receptor and GLP-1 receptor work together to reduce appetite, increase feelings of fullness, and slow how quickly food leaves your stomach. This dual mechanism is what distinguishes Mounjaro from older GLP-1 receptor agonist medications like Wegovy (semaglutide), which activate only the GLP-1 pathway.
The SURMOUNT-1 clinical trial showed that participants on Mounjaro lost an average of:
- 5 mg dose: approximately 15% of body weight
- 10 mg dose: approximately 19.5% of body weight
- 15 mg dose (maximum): approximately 20.9% of body weight
These results were achieved over 72 weeks alongside a reduced-calorie diet and increased physical activity. It's important to note that individual responses vary — some people lose more, some less. Weight loss is not assured, and results depend on adherence to lifestyle changes, dose, and individual factors.
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 Access, Eligibility, and What to Expect
In the UK, Mounjaro is available as a weekly weight loss injection administered using the Mounjaro KwikPen. You can access it through NHS specialist weight management services (Tier 3) if referred by your GP, though waiting lists vary significantly by region. Private access is also available through a clinical assessment with a UK-licensed prescriber, without needing an NHS referral.
You may be eligible if you have a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. Doses start at 2.5 mg and increase gradually to a maximum of 15 mg weekly. Common side effects include nausea, diarrhoea, and constipation, which often improve over time. Use our GLP-1 weight loss calculator to estimate potential outcomes, and our BMI calculator to check where you currently stand.
- 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's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) activates both GIP and GLP-1 receptors — a dual mechanism. Wegovy (semaglutide) activates only the GLP-1 receptor. In clinical trials, Mounjaro showed slightly higher average weight loss (15–21% vs approximately 15% for Wegovy), but both are effective. The choice depends on individual response and prescriber assessment.
Is Mounjaro available on the NHS in the UK?
Yes, Mounjaro is approved by NICE for use within NHS specialist weight management services. However, access varies by region and availability can be limited. Your GP can refer you to your local tier 3 weight management service. Private access is also available through clinical assessment without an NHS referral.
What are the most common side effects of Mounjaro?
The most common side effects are nausea, diarrhoea, vomiting, constipation, and stomach pain — these affect more than 1 in 10 people. Most improve over time as your body adjusts. Less common side effects include dizziness, fatigue, and injection site reactions. Always report persistent or severe symptoms to your healthcare provider.
How much weight will I lose on Mounjaro?
Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks, but individual results vary significantly. Some people lose more, some less — it depends on your starting weight, adherence to diet and exercise, dose, and individual factors. Weight loss is not assured. Your prescriber can discuss realistic expectations for your situation.
Can I use Mounjaro alongside other medications?
Mounjaro may interact with other medications, particularly oral contraceptives (which may be less effective) and diabetes medications. Always inform your prescriber of all medications you take. If you're on oral contraceptives, you may need a barrier method for 4 weeks after starting and after each dose increase.
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.