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 Tirzepitide? A UK Health Information Guide
Tirzepitide is a prescription-only weight management medication licensed in the UK under the brand name Mounjaro. It works by mimicking two natural hormones (GIP and GLP-1) that help regulate appetite and blood sugar. Clinical trials showed that people using tirzepitide alongside a reduced-calorie diet and increased physical activity lost an average of 15–21% of their body weight, though individual results vary significantly. This guide explains how tirzepitide works, who may be eligible, and what to expect from treatment in the UK health context.
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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 Tirzepitide Works and UK Eligibility
Mechanism of action: Tirzepitide is a dual GIP and GLP-1 receptor agonist — meaning it activates two hormone pathways in the body. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are natural hormones that help regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. Tirzepitide also slows gastric emptying (how quickly food leaves the stomach), which contributes to prolonged satiety.
UK eligibility: You may be eligible for tirzepitide (Mounjaro) in the UK if you have a BMI of 30 kg/m² or above, or a BMI of 27–29.9 kg/m² with at least one weight-related health condition. These conditions include type 2 diabetes, prediabetes, high blood pressure, abnormal blood fats, obstructive sleep apnoea, or a history of cardiovascular disease. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower thresholds may apply due to higher health risks at lower BMI levels. Use our free BMI calculator to check your personal BMI instantly.
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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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Tirzepitide Doses, Side Effects, and Practical Information
Dosing schedule: Tirzepitide is injected once weekly, starting at 2.5 mg and increasing gradually (typically every 4 weeks) to a maximum of 15 mg per week. Your prescriber will determine the right dose for you based on your individual response and tolerance. Each pen contains 4 weeks' supply and should be stored in a refrigerator at 2–8°C before use, then kept at room temperature (below 30°C) for up to 30 days after opening.
Common side effects: Nausea, diarrhoea, vomiting, and constipation are very common, especially when starting or increasing the dose. These typically improve over time. Less common effects include fatigue, dizziness, injection site reactions, and hair loss. Serious side effects requiring urgent medical attention include severe stomach pain (possible pancreatitis), gallbladder problems, or serious allergic reactions. Your GP or prescriber should assess whether tirzepitide is suitable for you before starting.
- 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 tirzepitide the same as semaglutide (Wegovy)?
No. Tirzepitide (Mounjaro) and semaglutide (Wegovy) are different medications. Tirzepitide is a dual GIP and GLP-1 agonist, while semaglutide is a GLP-1 agonist only. In clinical trials, tirzepitide showed slightly higher average weight loss (15–21% body weight) compared to semaglutide (approximately 15%), though individual results vary. Both are prescription-only and require clinical assessment before starting.
Can I get tirzepitide (Mounjaro) on the NHS in the UK?
Tirzepitide is available through NHS specialist tier 3 weight management services in some areas, following NICE approval. However, NHS access varies significantly by region and waiting times can be lengthy. Private access is available through GPhC-registered online pharmacies following a clinical assessment from a UK-licensed prescriber, without requiring an NHS referral.
What happens if I stop taking tirzepitide?
If you stop tirzepitide, appetite-suppressing effects typically wear off within days to weeks, and you may experience increased hunger and cravings. Weight regain is common if lifestyle changes (diet and physical activity) are not maintained. Your prescriber can advise on the safest way to stop treatment if needed.
Who cannot take tirzepitide?
Tirzepitide is not suitable if you are under 18, pregnant, planning pregnancy, or breastfeeding. It's also contraindicated if you have a personal or family history of medullary thyroid cancer, multiple endocrine neoplasia type 2, severe gastroparesis, or allergy to tirzepitide. Your prescriber will assess contraindications during a clinical consultation.
How long does it take to see weight loss results with tirzepitide?
Weight loss typically begins within the first few weeks but increases gradually over months as your dose escalates. Most significant weight loss occurs between weeks 12–68 of treatment. Use our GLP-1 weight loss calculator to estimate potential results based on clinical trial data, though your individual results will depend on adherence to dietary changes and physical activity alongside medication.
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.