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
Free tracker included with every download this week
What's in Mounjaro? Active Ingredient & Composition
Mounjaro is a prescription-only weight management injection containing tirzepatide as its active ingredient. Tirzepatide is a dual GIP and GLP-1 receptor agonist — a medication that works differently from other weight loss injections by activating two natural hormone pathways rather than one. It's manufactured by Eli Lilly and approved by the MHRA for use in the UK. Understanding what's inside Mounjaro helps explain how it works and what to expect during treatment.
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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.
What Tirzepatide Does — The Active Ingredient Explained
Tirzepatide is the active pharmaceutical ingredient in Mounjaro. Unlike single-action GLP-1 medications like Wegovy, tirzepatide activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptor pathways. This dual mechanism may help regulate appetite and food cravings more effectively than single-action medications alone.
When you inject Mounjaro, tirzepatide:
- Signals to your brain that you are fuller after eating smaller portions
- Slows how quickly food leaves your stomach (gastric emptying)
- Helps regulate how your body uses fat
- Supports blood sugar control
In the SURMOUNT-1 clinical trial, participants using Mounjaro at maximum dose (15mg) lost an average of approximately 20.9% of their body weight over 72 weeks, though individual results vary significantly. Clinical trials show results at the 5mg dose (approximately clinically studied weight loss) and 10mg dose (approximately clinically studied weight loss). Calculate your potential weight loss range using our free GLP-1 weight loss calculator above.
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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.
Mounjaro Formulation & How It's Delivered in the UK
Mounjaro is supplied as a pre-filled KwikPen auto-injection device. Each pen contains one weekly dose and delivers the medication as a clear, colourless solution via subcutaneous injection (under the skin). Each KwikPen box contains four pre-filled pens, providing a month's supply.
The injection can be administered to the stomach (abdomen), upper thigh, or upper arm. Unopened pens must be stored in a refrigerator at 2–8°C. After first use, pens can be kept at room temperature (below 30°C) for up to 30 days. In the UK, Mounjaro is available via NHS specialist tier 3 weight management services or through private prescription following an online clinical assessment with a UK-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. Tirzepatide (Mounjaro) is a dual GIP + GLP-1 agonist, while semaglutide (Wegovy) is a single GLP-1 agonist. Both are prescription-only weight management injections approved in the UK, but they work through different mechanisms. Clinical trials show different average weight loss outcomes, though results vary between individuals. Your GP or prescriber can advise which may be suitable for you.
What are the inactive ingredients in Mounjaro?
The Mounjaro KwikPen solution contains tirzepatide plus inactive ingredients including sodium chloride, phosphate buffer, and water for injection. The full list of inactive ingredients is available in the patient information leaflet supplied with your prescription. If you have allergies to any ingredients, inform your prescriber before starting treatment.
Can I use Mounjaro if I'm allergic to GLP-1 medications?
No. Because tirzepatide activates GLP-1 receptors as part of its dual mechanism, it should not be used if you've previously had an allergic reaction to GLP-1 medications or any ingredient in Mounjaro. Always disclose your full medical history and previous medication reactions to your prescriber before starting any weight management treatment.
How does tirzepatide differ from Ozempic or Saxenda?
Ozempic (semaglutide) is licensed for type 2 diabetes, not weight management. Saxenda (liraglutide) is a daily GLP-1 injection for weight management. Mounjaro (tirzepatide) is a once-weekly dual GIP + GLP-1 injection. All are prescription-only. Clinical trial results differ between these medications. Your prescriber can discuss which option may be appropriate following your assessment.
Does Mounjaro contain insulin?
No. Mounjaro is not insulin and does not work like insulin. It's a receptor agonist that activates natural hormone pathways to help regulate appetite and food intake. If you have type 2 diabetes and take insulin, Mounjaro may affect how much insulin you need — always inform your prescriber of all current medications before starting Mounjaro.
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.