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?
Tirzepatide is a prescription-only weight management injection licensed in the UK under the brand name Mounjaro. It's a dual GIP and GLP-1 receptor agonist—meaning it works in two ways to help regulate appetite and support weight management alongside diet and exercise. Unlike some other weight loss medications, tirzepatide activates both the GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 hormone pathways, which may help you feel fuller after eating smaller portions. Clinical trials have shown promising results, though individual outcomes vary significantly. Your GP or a licensed prescriber can assess whether tirzepatide is appropriate 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.
How Tirzepatide Works in Your Body
Tirzepatide is a dual GIP and GLP-1 receptor agonist—this dual mechanism is what sets it apart from other weekly weight loss injection medications like semaglutide (Wegovy). When you take tirzepatide, it mimics two natural hormones your gut produces after eating: GIP and GLP-1.
These hormones signal your brain that you're full, which may help reduce appetite and food cravings. Tirzepatide also slows how quickly food leaves your stomach (gastric emptying), allowing you to feel satisfied after smaller meals. Additionally, it helps your body regulate blood sugar and fat utilisation.
- Activates both GIP and GLP-1 pathways (dual action)
- Signals fullness to the brain
- Reduces food cravings and appetite
- Slows stomach emptying
- Supports blood sugar regulation
It's important to note that tirzepatide is not insulin and does not work the same way as insulin therapy. It's a prescription-only medicine requiring a clinical assessment before use.
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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.
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Tirzepatide in the UK: Doses, Eligibility, and Results
In the UK, tirzepatide (Mounjaro) comes as a pre-filled KwikPen for once-weekly injections. Doses range from 2.5 mg (starting) up to 15 mg (maximum). Your prescriber will typically start you at the lowest dose and increase gradually every 4 weeks based on how you respond.
Eligibility for tirzepatide in the UK includes adults with a BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Ethnicity-adjusted thresholds may apply—South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI levels. In clinical trials, participants using tirzepatide lost an average of 15–21% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. Results vary significantly between individuals.
- 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, and Mounjaro is the brand name. You may hear both terms used interchangeably in the UK. Tirzepatide is a GIP and GLP-1 receptor agonist manufactured by Eli Lilly.
How is tirzepatide different from semaglutide (Wegovy)?
Tirzepatide uses a dual mechanism—activating both GIP and GLP-1 pathways—whereas semaglutide (Wegovy) is a GLP-1-only agonist. Clinical trials suggest tirzepatide may result in slightly greater average weight loss, though individual results vary. Both are once-weekly injections and prescription-only in the UK.
Can I get tirzepatide on the NHS?
Tirzepatide is available through NHS specialist weight management services (tier 3), but access varies significantly by region and availability. Waiting lists can be lengthy. Private prescribing is also available through UK-licensed online prescribers following a clinical assessment. Your GP can advise on local NHS options.
What are the main side effects of tirzepatide?
Very common side effects include nausea, diarrhoea, vomiting, and stomach pain. These often decrease as your body adjusts. Less common side effects include fatigue, dizziness, and injection site reactions. Serious side effects are rare but may include severe stomach pain (possible pancreatitis) or gallbladder issues. Always report persistent symptoms to your prescriber.
How do I calculate my potential weight loss with tirzepatide?
Our free GLP-1 weight loss calculator can help you estimate potential weight loss outcomes at different doses based on clinical trial data. Use our BMI calculator alongside it to understand your starting BMI and healthy weight range. Remember that individual results vary—your prescriber can give personalised guidance.
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.