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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What Is Tirzepatide? A Complete UK Guide
Tirzepatide is a prescription-only weight loss injection licensed in the UK under the brand name Mounjaro. It's a dual GIP and GLP-1 receptor agonist, meaning it works by activating two natural hormone pathways in your body to help reduce appetite and support weight management. If you're considering tirzepatide or want to understand how it differs from other weight loss medications, this guide explains everything you need to know about this weekly injection, how it works, and who may be eligible in the UK.
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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 Tirzepatide Works as a Dual Receptor Agonist
Tirzepatide is unique because it activates two hormone pathways simultaneously — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) — making it a dual-action medication. This dual mechanism may help regulate appetite more effectively than single-action weight loss medications. When you take tirzepatide, it signals to your brain that you're fuller after eating smaller portions, slows how quickly food leaves your stomach, and supports better blood sugar management. Unlike insulin, tirzepatide is not a blood sugar medication — it's a weight management injection. The most common brand name in the UK is Mounjaro, delivered via a convenient weekly KwikPen auto-injector. Clinical trials showed that tirzepatide helped participants lose an average of 15–21% of their body weight over 72 weeks, though individual results vary considerably. Many people notice reduced food cravings and increased feelings of fullness within the first few weeks, though side effects like nausea are common initially.
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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
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Tirzepatide Dosing, Eligibility & Access in the UK
In the UK, tirzepatide dosing starts at 2.5 mg weekly and escalates gradually over 16+ weeks to a maximum of 15 mg. Your prescriber adjusts the dose based on your response and tolerability. Eligibility typically includes adults with a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions like type 2 diabetes, high blood pressure, or cardiovascular disease. Lower BMI thresholds (starting at 27.5) may apply if you're of South Asian, Black African, Black Caribbean, or Chinese heritage, as these groups face higher health risks at lower BMI levels. Use our free BMI calculator to check your personal BMI instantly. Access is available through NHS specialist weight management services (tier 3) or private prescribing following an online clinical assessment. Always consult your GP or a licensed prescriber to confirm whether tirzepatide is appropriate for you.
- 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 Mounjaro?
Yes. Tirzepatide is the active ingredient; Mounjaro is the brand name under which it's marketed in the UK. Mounjaro is delivered via a KwikPen auto-injector and is the only tirzepatide formulation licensed for weight management in the UK.
How does tirzepatide differ from other weight loss injections like Wegovy?
Tirzepatide is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 receptor agonist only. This means tirzepatide has two mechanisms of action. Clinical trials showed tirzepatide achieved average weight loss of 15–21%, compared to approximately 15% with Wegovy, though individual results vary significantly.
Can I get tirzepatide on the NHS in the UK?
Tirzepatide (Mounjaro) is available through NHS specialist tier 3 weight management services, but availability varies significantly by region and waiting lists can be lengthy. Alternatively, tirzepatide is available through private prescribing following an online clinical assessment with a UK-licensed prescriber.
What are the common side effects of tirzepatide?
Very common side effects include nausea, diarrhoea, vomiting, and constipation — especially when starting or increasing doses. These often improve over time. Less common effects include headache, fatigue, and injection site reactions. Serious side effects are rare but require urgent medical attention if experienced.
How quickly will I lose weight on tirzepatide?
Weight loss timing varies. Some people notice appetite reduction within days; others take weeks. Average weight loss in clinical trials was 0.5–1 kg per week, but this depends on your diet, activity level, starting weight, and individual metabolism. Our GLP-1 weight loss calculator can help estimate potential outcomes at different doses.
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.