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 Injection and How Does It Work?
Mounjaro is a prescription-only weekly weight loss 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 by activating two natural hormone pathways to help regulate appetite and support weight management alongside diet and physical activity. Understanding how this medication works, who can access it, and what to expect is essential before considering it as part of your weight loss journey.
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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 the Mounjaro Injection Works and Key Features
Mounjaro is unique because it's a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways rather than one. This dual mechanism may help regulate appetite more effectively than single-action medications. The injection works by:
- Signalling to your brain that you're full, reducing food cravings and portion sizes
- Slowing how quickly food leaves your stomach, supporting longer-lasting satiety
- Helping your body regulate fat utilisation
- Stabilising blood sugar levels
Mounjaro comes as a pre-filled KwikPen device — a weekly subcutaneous injection you can self-administer at home. Each pen contains four weekly doses. Clinical trials showed that participants on Mounjaro lost an average of 15–21% of their body weight over 72 weeks, though individual results vary significantly. Weight loss is most effective when combined with a reduced-calorie diet and increased physical activity.
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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.
UK Eligibility, Doses, and How to Access Mounjaro
In the UK, Mounjaro may be prescribed to adults with a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. People of South Asian, Chinese, Black African, or Black Caribbean heritage may qualify at lower BMI thresholds. Doses start at 2.5 mg and escalate weekly over 16+ weeks to a maximum of 15 mg, depending on tolerability and response.
Access in the UK is available through NHS specialist tier 3/4 weight management services (via GP referral, though waiting lists vary by region) or through private online prescribing following a clinical assessment with a UK-licensed prescriber. Our GLP-1 weight loss calculator can help you estimate potential outcomes at different doses.
In the UK, Mounjaro may be prescribed to adults with a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. People of South Asian, Chinese, Black African, or Black Caribbean heritage may qualify at lower BMI thresholds. Doses start at 2.5 mg and escalate weekly over 16+ weeks to a maximum of 15 mg, depending on tolerability and response.
Access in the UK is available through NHS specialist tier 3/4 weight management services (via GP referral, though waiting lists vary by region) or through private online prescribing following a clinical assessment with a UK-licensed prescriber. Our GLP-1 weight loss calculator can help you estimate potential outcomes 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.
Is Mounjaro injection the same as Wegovy or Ozempic?
No. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) activates only GLP-1. Both are licensed for weight management in the UK. Ozempic also contains semaglutide but is licensed for type 2 diabetes only — not for weight loss. Only Wegovy is the licensed semaglutide formulation for weight management.
How much weight can I expect to lose with Mounjaro injection?
Clinical trials showed an average weight loss of 15–21% of body weight over 72 weeks on Mounjaro, but individual results vary significantly. Some people lose more, others less. Weight loss depends on your starting weight, dose, diet, activity level, and how your body responds. A prescriber can discuss realistic expectations during your assessment.
What are the most common side effects of Mounjaro injection?
The most common side effects are nausea, diarrhoea, vomiting, and constipation — especially when starting or increasing doses. These often improve over time. Injection site reactions, fatigue, and reduced appetite also occur. Serious side effects are rare but include severe stomach pain (pancreatitis) or gallbladder problems. Report any concerning symptoms to your prescriber.
Can I get Mounjaro injection on the NHS in the UK?
Yes, Mounjaro is approved by NICE for NHS use through specialist tier 3/4 weight management services. Access is via GP referral, but availability varies significantly by region and waiting lists can be lengthy. Private access is available through online prescribing services following clinical assessment — you don't need an NHS referral to access it privately.
How do I store my Mounjaro injection pens at home?
Unopened Mounjaro pens must be refrigerated at 2–8°C and kept in their original carton away from light. Do not freeze. Once you've started using a pen, you can keep it at room temperature (below 30°C) for up to 30 days. Dispose of the pen 30 days after first use, even if it still contains medication.
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.