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 Slimming Injection?
Mounjaro is a prescription-only weekly weight loss injection available in the UK that works differently from other slimming injections on the market. It contains tirzepatide, a dual GIP and GLP-1 receptor agonist, which means it activates two natural hormone pathways to help regulate appetite and support weight management. This dual mechanism sets it apart from single-action GLP-1 injections like Wegovy. Mounjaro is delivered via a convenient pre-filled KwikPen device and requires a clinical assessment before prescription. It's used alongside a reduced-calorie diet and increased physical activity for best results.
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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 What Makes It Different
Mounjaro's dual-action mechanism is what distinguishes it from other weight loss injections. It activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptor pathways — two natural hormone systems that regulate hunger and satiety. This combined effect may help you feel fuller after smaller meals, reduce cravings, and support better appetite control throughout the day.
The injection slows how quickly food leaves your stomach, which can prolong feelings of fullness. Unlike some other slimming injections, Mounjaro starts at a lower dose (2.5 mg) and is gradually increased weekly over several weeks until reaching your maintenance dose, typically 5–15 mg per week depending on your individual response and tolerance.
- Once-weekly injection — administered at home using the pre-filled KwikPen
- Dual hormone pathway activation — GIP + GLP-1 action (vs single GLP-1 in Wegovy)
- Gradual dose escalation — minimum 4 weeks at each dose before increasing
- Prescription-only — requires assessment by a licensed prescriber
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 and How to Access It in the UK
In the UK, you may be eligible for Mounjaro if you have a BMI of 30 or above, or a BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds apply due to higher health risks at lower weights.
You can access Mounjaro through two routes: NHS specialist services (tier 3 weight management services) following a GP referral, or private prescribing via a licensed UK prescriber following an online clinical assessment. Check your BMI using our free BMI calculator to see where you stand, and discuss options with your GP or a licensed prescriber to determine the best route for you. Access and waiting times vary significantly by region and service availability.
- 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-only weekly weight loss injections, but Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials showed Mounjaro achieved average weight loss of approximately 15–21% body weight, while Wegovy achieved around 15%. Both work best alongside diet and exercise, and individual results vary.
What are the main side effects of Mounjaro?
Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain, particularly when starting or increasing doses. These often reduce over time. Less common side effects include fatigue, dizziness, and injection site reactions. Serious side effects are rare but include severe stomach pain (possible pancreatitis) — seek immediate medical help if this occurs.
How much weight can I expect to lose with Mounjaro?
In clinical trials, people using Mounjaro lost an average of 15–21% of their body weight over 72 weeks, though individual results vary significantly. Your GLP-1 weight loss calculator can help estimate potential outcomes at different doses. Weight loss depends on your starting weight, adherence to a reduced-calorie diet, physical activity level, and how your body responds to the medication.
How do I get Mounjaro prescribed in the UK?
Through NHS specialist weight management services (tier 3) via GP referral, or through private prescribing following an online clinical assessment. NHS access varies by region and may involve waiting lists. Private options are available without NHS referral. A licensed prescriber will assess your eligibility based on BMI, health conditions, and medical history.
Does Mounjaro work if I don't change my diet?
No. Mounjaro is designed to be used alongside a reduced-calorie diet and increased physical activity. Clinical trials showed weight loss only occurred when participants combined the injection with lifestyle changes. The medication supports appetite control, but sustainable weight loss requires both medical support and commitment to healthier 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.