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
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When Was Mounjaro Invented
Mounjaro (tirzepatide) was invented and developed by pharmaceutical company Eli Lilly and approved for weight management in the UK by the MHRA in November 2023. The medication represents a innovative in weight loss treatment as a dual GIP and GLP-1 receptor agonist — a different mechanism to earlier single-action GLP-1 medications like Wegovy. Understanding Mounjaro's development and how it differs from other weight loss injections can help you understand your treatment options if you're considering prescription weight management support.
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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.
Development and Clinical Trials
Mounjaro's development timeline: Tirzepatide was originally researched and developed by Eli Lilly as a potential treatment for type 2 diabetes, with early research beginning in the early 2010s. The company recognised the medication's potential for weight management and conducted the SURMOUNT clinical trial programme specifically to assess its effectiveness for weight loss. The SURMOUNT-1 trial, the primary study, showed participants using the maximum 15mg dose lost an average of approximately 21% of their body weight over 72 weeks — significantly more than earlier GLP-1 medications like semaglutide (Wegovy). Based on these results, Eli Lilly submitted tirzepatide for regulatory approval across multiple countries.
Why dual action matters: Unlike Wegovy, which is a single GLP-1 receptor agonist, Mounjaro activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 pathways simultaneously. This dual mechanism may explain the greater average weight loss observed in clinical trials, though individual results vary significantly.
Clinically BackedWeight Loss
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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 Approval and Prescription Access
The MHRA (Medicines and Healthcare products Regulatory Authority) approved Mounjaro for weight management in November 2023, followed by NICE (National Institute for Health and Care Excellence) approval for NHS use within specialist weight management services in early 2024. In the UK, Mounjaro is available as a prescription-only medicine through the NHS via tier 3 specialist weight management services, or privately through online prescribing following a clinical assessment. Access varies significantly by region — some areas have lengthy waiting lists for NHS specialist services. The Mounjaro KwikPen (the pre-filled injection device) comes in doses from 2.5mg up to 15mg, with weekly dose adjustments typically made over 4-week periods. If you're considering Mounjaro, your GP can refer you to specialist services or you can access private prescribing through a UK-licensed prescriber.
- 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 is tirzepatide and how does it differ from semaglutide?
Tirzepatide (Mounjaro) is a dual GIP and GLP-1 receptor agonist, meaning it activates two different hormone pathways. Semaglutide (Wegovy) activates only the GLP-1 pathway. Clinical trials suggest tirzepatide may produce slightly greater average weight loss, though individual results vary significantly. Both are prescription-only weight management medications available in the UK.
When did Mounjaro become available in the UK?
Mounjaro received MHRA approval in November 2023 and NICE approval for NHS use in early 2024. Private access through online prescribing began shortly after regulatory approval. Availability through NHS specialist services continues to vary by region due to demand and service capacity.
How does Mounjaro's weight loss compare to Wegovy?
In the SURMOUNT-1 trial, Mounjaro 15mg showed average weight loss of approximately 21% of body weight. Wegovy (semaglutide) showed average weight loss of approximately 15% in the STEP-1 trial. However, individual results vary greatly, and clinical trial averages should not be used to predict personal outcomes. Your prescriber can discuss which medication might suit you.
Is Mounjaro available on the NHS in the UK?
Yes, Mounjaro is available through NHS specialist tier 3 weight management services for eligible adults. However, access varies significantly by region and waiting lists can be lengthy. Private prescribing through UK-licensed prescribers offers an alternative route. Your GP can provide guidance on NHS eligibility and referral pathways in your area.
What is a Mounjaro KwikPen and how does it work?
The Mounjaro KwikPen is a pre-filled, disposable injection device supplied by Eli Lilly. Each pen contains four weekly doses. You inject once per week under the skin (stomach, thigh, or upper arm). Doses start at 2.5mg and typically increase every four weeks up to a maximum of 15mg, guided by your prescriber.
Ready to take control of your weight loss journey?
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