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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Mounjaro Injection Before And After — What The Evidence Shows
When considering Mounjaro (tirzepatide) as a weekly weight loss injection in the UK, many people want to understand what realistic results look like. Clinical trials and NHS specialist services have documented measurable weight loss outcomes, though individual results vary significantly. This guide explains what the evidence actually shows about Mounjaro's effects, how NHS eligibility works, and what to realistically expect from tirzepatide treatment in a UK context.
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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.
NHS Eligibility And Clinical Trial Results
NHS access to Mounjaro for weight management: In the UK, tirzepatide (Mounjaro) is available through specialist tier 3 weight management services following NICE approval. You cannot access Mounjaro through a standard GP prescription — you must be referred to a specialist NHS weight management service by your GP.
NHS eligibility criteria: Adults with a BMI of 30 kg/m² or above qualify, or BMI 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 (27.5 and above).
Clinical trial data (SURMOUNT-1): In the main clinical trial, participants on the maximum 15 mg dose lost an average of approximately 20.9% of their body weight over 72 weeks alongside reduced-calorie diet and increased physical activity. At 10 mg, average loss was around 19.5%. At 5 mg, approximately 15%. These are averages — some participants lost more, others less.
- Weight loss typically occurs gradually across weeks 5–20 as doses are increased
- Maximum benefit usually seen by week 20–24 of treatment
- Loss continues steadily through the trial period at maintained doses
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.
NHS Versus Private Access And Realistic Expectations
NHS vs private routes in the UK: NHS specialist weight management services have long waiting lists in many regions — sometimes months to years. Private online prescribing services offer faster access following online clinical assessment by a UK-licensed prescriber, but you will need to pay out-of-pocket (typically £129–£320+ monthly depending on dose). Both routes require clinical assessment before prescription.
Important realities about "before and after": Individual results vary based on genetics, baseline metabolism, diet adherence, physical activity, and other medications. A person who loses 20% body weight will see noticeable changes; someone losing 10% may see modest changes. Weight loss injections work alongside lifestyle changes, not instead of them. Use our free weight loss calculator to estimate potential outcomes at different Mounjaro doses, and our BMI calculator to understand your current health baseline.
- 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.
Does Mounjaro cause dramatic weight loss overnight?
No. Weight loss with Mounjaro is gradual. Clinical trials show meaningful loss typically appears from week 5 onwards as doses are increased. Maximum effects are usually seen by weeks 20–24. The injection works by reducing appetite and supporting fullness — sustainable loss takes time alongside diet and exercise changes.
Can I get Mounjaro on the NHS in the UK?
Mounjaro (tirzepatide) is available on the NHS through specialist tier 3 weight management services. You cannot get it from a standard GP — you need a referral. NICE has approved it for use in specialist services. Waiting lists vary by region and can be lengthy. Private online prescribing offers faster access but requires out-of-pocket payment.
What percentage of body weight do people lose with Mounjaro?
In the SURMOUNT-1 clinical trial, participants lost on average 15–21% of body weight depending on dose, alongside diet and exercise changes. However, individual results vary significantly. Some lose more, others lose less. Weight loss also depends on your starting weight, lifestyle adherence, metabolism, and other personal factors.
Are before-and-after claims from Mounjaro users reliable?
Individual testimonials vary widely and are not a reliable indicator of your personal results. Clinical trial data is more reliable because it measures average outcomes across hundreds of participants. However, even trial averages hide significant individual variation. Your GP or prescriber can give you realistic expectations based on your own health profile.
What happens to weight after stopping Mounjaro?
Clinical trials show that weight typically begins to return if you stop Mounjaro without maintaining the lifestyle changes that supported initial loss. The injection is not a permanent solution — it's a tool to support weight loss while you build sustainable eating and activity habits. Long-term management usually requires ongoing treatment or sustained behaviour change.
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.