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 Works — The Dual Hormone Mechanism
Tirzepatide is a prescription-only weight management injection that works differently from other weight loss medications available in the UK. Unlike single-action GLP-1 receptor agonists, tirzepatide activates two natural hormone pathways in your body — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help explain why clinical trials showed promising weight loss results. The medication is sold under the brand name Mounjaro as a once-weekly injection delivered via a KwikPen device. Understanding how tirzepatide works can help you make an informed decision about whether this treatment might be suitable for you.
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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.
The Dual Hormone Mechanism — GIP and GLP-1 Explained
Tirzepatide works by mimicking two natural hormones your gut produces after eating. GLP-1 (glucagon-like peptide-1) is well-established in weight management — it signals fullness to your brain, reduces hunger, and slows how quickly food leaves your stomach. GIP (glucose-dependent insulinotropic polypeptide) is the newer addition — it also plays a role in appetite regulation and may help with how your body processes and stores fat.
When you inject tirzepatide once weekly, it activates both pathways simultaneously. This dual action is what distinguishes tirzepatide from other weight loss injections like semaglutide (Wegovy), which only targets GLP-1. In the SURMOUNT-1 clinical trial, participants using tirzepatide experienced an average weight loss of 15–21% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. Individual results vary significantly, and weight loss is not assured — but the dual mechanism may contribute to its effectiveness.
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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.
Effects on Appetite, Digestion, and Metabolism
Appetite suppression: Tirzepatide reduces hunger signals and food cravings, helping you feel satisfied after smaller meals. This may make it easier to stick to a reduced-calorie diet.
Slower stomach emptying: The medication slows gastric emptying — how quickly food moves from your stomach to your intestines. This extends the feeling of fullness. Note: this effect typically reduces over time as your body adapts.
Fat metabolism: GIP activation may help your body better regulate how it stores and uses fat. This is an active area of research and may partly explain why tirzepatide users report different results than GLP-1-only medications. You can use our free GLP-1 weight loss calculator to see projected results at different tirzepatide doses based on clinical trial data.
- 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 insulin?
No. Tirzepatide is not insulin. It's a GIP and GLP-1 receptor agonist — it mimics natural hormones your gut produces. It does not contain insulin and does not work in the same way. If you have type 2 diabetes, tirzepatide may help with blood sugar control as a side effect, but it's not an insulin replacement.
How quickly does tirzepatide start working?
You may notice reduced appetite within the first week, but most weight loss becomes apparent after 4–8 weeks. Clinical trials measured weight loss over 72 weeks. Your GP or prescriber will review your progress at regular intervals and may adjust your dose if needed.
Why is tirzepatide given as a once-weekly injection?
The medication is designed to work effectively with once-weekly dosing, making it more convenient than daily injections. You inject under the skin (subcutaneously) in your abdomen, thigh, or upper arm using the pre-filled KwikPen device. Rotation of injection sites is recommended.
Can tirzepatide stop working over time?
Some users report that appetite-suppressing effects may reduce slightly as the body adapts. However, weight loss typically continues if you maintain your reduced-calorie diet and physical activity. Your prescriber can adjust your dose if needed — tirzepatide doses range from 2.5 mg up to 15 mg weekly.
How does tirzepatide compare to other weight loss injections?
Tirzepatide (Mounjaro) uses a dual GIP and GLP-1 mechanism, while semaglutide (Wegovy) is GLP-1-only. Clinical trials suggest tirzepatide may produce slightly higher average weight loss, but individual results vary. Both are prescription-only and require clinical assessment. Your GP or prescriber can advise which is more suitable for you.
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.