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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How Tirzepatide Supports Weight Loss
Tirzepatide is a prescription-only weekly injection that may help support weight loss in eligible UK adults. It works differently from other weight loss medications by activating two natural hormone pathways — GIP and GLP-1 — which help regulate appetite, reduce food cravings, and support feeling fuller after meals. Clinical trials showed an average weight loss of 15–21% of body weight, though individual results vary significantly. This guide explains how tirzepatide works, what to expect, and how to access it in the UK.
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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 Tirzepatide Works for Weight Loss
Tirzepatide is a dual GIP and GLP-1 receptor agonist — this dual mechanism is what sets it apart from other weight loss injections like semaglutide. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are natural hormones your body produces after eating.
- GLP-1 signals to your brain that you are full, helping reduce appetite and food cravings
- GIP works alongside GLP-1 to enhance feelings of satiety and support appetite regulation
- Together, these pathways slow how quickly food leaves your stomach, helping you feel fuller for longer
- Tirzepatide also supports your body's natural fat utilisation processes
The weekly injection (brand name Mounjaro or via generic tirzepatide) is given once every 7 days using a pre-filled KwikPen device. You inject it under the skin in the stomach, thigh, or upper arm. Calculate your potential weight loss outcomes using our free GLP-1 weight loss calculator above — results vary based on dose, diet, and lifestyle changes.
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.
Weight Loss Results and UK Access
In the SURMOUNT-1 clinical trial, participants taking tirzepatide achieved an average weight loss of approximately 15% at 5mg dose, 19.5% at 10mg, and 20.9% at the maximum 15mg dose over 72 weeks. These results were achieved alongside a reduced-calorie diet and increased physical activity — medication works best combined with lifestyle changes.
In the UK, tirzepatide is available through NHS specialist weight management services (tier 3/4) following GP referral, though waiting lists vary significantly by region. It is also available privately through GPhC-registered online prescribing services following a clinical assessment. Eligibility typically requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Use our free BMI calculator above to check your current BMI, then discuss options with your GP or a 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.
Is tirzepatide the same as semaglutide?
No. Both are GLP-1-based weight loss injections, but tirzepatide is a dual GIP and GLP-1 agonist, while semaglutide (Wegovy) is GLP-1 only. Clinical trials show tirzepatide may support slightly higher average weight loss, though individual results vary. Both are prescription-only and require clinical assessment.
How much weight can I expect to lose with tirzepatide?
Clinical trials showed an average loss of 15–21% of body weight over 72 weeks at maximum dose, alongside diet and activity changes. However, individual results vary considerably — some people lose more, some less. Your GP or prescriber can discuss realistic expectations for your situation.
How do I access tirzepatide weight loss in the UK?
Through NHS specialist tier 3 weight management services (referral from your GP) or privately via online prescribing following a clinical assessment. NHS access varies by region and may have waiting lists. Private options are available without NHS referral through GPhC-registered pharmacies.
What are the common side effects of tirzepatide?
Very common side effects include nausea, diarrhoea, constipation, and stomach pain — usually mild and improve over time as your body adjusts. Less common effects include vomiting, indigestion, fatigue, and injection site reactions. Serious side effects are rare but require urgent medical attention (severe stomach pain, allergic reactions).
Can I use tirzepatide if I'm also taking other medications?
Tirzepatide may affect how your body absorbs some oral medications, particularly oral contraceptives — additional contraception may be needed. If you take diabetes medications, your prescriber may adjust doses. Always inform your prescriber of all medications and supplements before starting tirzepatide.
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.