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 Is In The Mounjaro Injection?
Mounjaro is a weekly weight loss injection containing tirzepatide, a dual GIP and GLP-1 receptor agonist โ meaning it activates two natural hormone pathways your body uses to regulate appetite and blood sugar. Unlike Wegovy, which contains semaglutide (a GLP-1 agonist only), Mounjaro's dual mechanism may help you feel fuller for longer and support weight management differently. Both are prescription-only medicines available in the UK following clinical assessment. Understanding what's inside your weekly weight loss injection helps you make informed decisions about your treatment options.
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Calculate Your BMI Instantly
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/mยฒ).
While BMI isn't a perfect measure โ it doesn't account for muscle mass, bone density, or fat distribution โ 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 โ 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.
Active Ingredient & Dual Mechanism Explained
Mounjaro contains tirzepatide, manufactured by Eli Lilly. Each KwikPen pre-filled injection pen delivers tirzepatide at doses ranging from 2.5 mg to 15 mg once weekly. The active ingredient works by stimulating two hormone receptors: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). In SURMOUNT clinical trials, this dual mechanism helped participants lose an average of 15โ21% of their body weight at maximum dose, though individual results vary significantly. The injection also contains sodium chloride, disodium phosphate dihydrate, potassium dihydrogen phosphate, and water for injection โ standard pharmaceutical stabilisers. Unlike Wegovy (semaglutide), which targets only the GLP-1 pathway, Mounjaro's dual GIP/GLP-1 action may provide additional appetite suppression and metabolic support, though prescriber assessment determines which is most suitable for you personally.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care โ from a UK-regulated online pharmacy.
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 โ 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 โ completely free.
Mounjaro vs Wegovy: What's The Difference?
Wegovy contains semaglutide, a GLP-1 receptor agonist only, whereas Mounjaro activates both GIP and GLP-1 pathways. In clinical trials, Wegovy participants (STEP trials) lost approximately 14.9% of body weight, compared to Mounjaro's 15โ21% in SURMOUNT trials โ though results vary between individuals and depend on dose, lifestyle changes, and individual response. Both are UK prescription-only medicines administered once weekly. Both require clinical assessment before prescribing and are typically used alongside reduced-calorie diet and increased physical activity. Your GP or private prescriber will assess which suits your individual health needs, medical history, and previous treatment response. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses for either medication, and consult your prescriber about which mechanism aligns best with your health goals.
- 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 โ 5mg โ 7.5mg โ 10mg โ 12.5mg โ 15mg
- GLP-1 receptor agonist
- Up to 15โ17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg โ 0.5mg โ 1mg โ 1.7mg โ 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 | 15โ22.5% | 12โ17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20โ32 weeks | 16โ20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | ยฃ150โยฃ250/month | ยฃ150โยฃ300/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 โ 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 Mounjaro the same as Wegovy?
No. Both are once-weekly weight loss injections, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. This means Mounjaro activates two hormone pathways, whereas Wegovy activates one. Clinical trials showed both support weight loss, but individual suitability differs โ your prescriber will assess which is appropriate for you.
What exactly is tirzepatide?
Tirzepatide is the active pharmaceutical ingredient in Mounjaro. It's a synthetic peptide that mimics and amplifies the effects of your natural GIP and GLP-1 hormones. These hormones regulate appetite, blood sugar, and how quickly food leaves your stomach. Tirzepatide is manufactured by Eli Lilly and is subject to UK MHRA additional monitoring (Black Triangle status).
How is Mounjaro supplied in the UK?
Mounjaro is supplied as a pre-filled KwikPen injection device for once-weekly self-administration. Each pen contains one dose (2.5 mg to 15 mg). Unopened pens must be refrigerated at 2โ8ยฐC. After first use, pens can be stored at room temperature (below 30ยฐC) for up to 30 days. Always follow your prescriber's storage instructions.
Does Mounjaro contain any allergens?
Mounjaro is generally well-tolerated, but inform your prescriber if you have allergies to tirzepatide or any ingredients in the injection. Tell your GP about any history of thyroid cancer, gallbladder problems, or severe stomach conditions, as these may affect suitability. Always declare all medications and allergies during your clinical assessment.
Can I switch from Wegovy to Mounjaro?
Switching between weight loss medications should only be managed by your prescriber. While approximate dose equivalences exist (1.7 mg Wegovy โ 10 mg Mounjaro), there is no official established conversion chart. Your prescriber will assess whether switching is appropriate and how to transition safely, monitoring your response and side effects throughout.
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.