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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Mounjaro vs Wegovy: Instructions and Key Differences
If you're considering weight loss injections in the UK, you may have encountered both Mounjaro and Wegovy. While both are prescription-only once-weekly injections licensed for weight management, they work differently and have different dosing schedules. This guide compares their mechanisms, clinical trial results, and practical administration to help you understand what your prescriber might recommend. Both medications require a clinical assessment before prescription, and individual suitability depends on your personal health profile.
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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 Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways simultaneously. This dual mechanism may help regulate appetite more effectively than single-pathway medications. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating one hormone pathway.
- Mounjaro: dual GIP + GLP-1 action; may signal fullness through two pathways
- Wegovy: GLP-1 only; single-pathway appetite regulation
- Both slow gastric emptying (food leaving the stomach)
- Both require lifestyle changes (reduced calories, increased activity) to be effective
In the SURMOUNT clinical trial, Mounjaro achieved average weight loss of approximately 20.9% at the maximum 15 mg dose over 72 weeks. In the STEP clinical trials, Wegovy achieved average weight loss of approximately 14.9% at the maximum 2.4 mg dose over 68 weeks. However, individual results vary significantly.
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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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Dosing Schedules and Practical Instructions
Mounjaro uses a KwikPen pre-filled device with doses starting at 2.5 mg and escalating every 4 weeks up to a maximum of 15 mg once weekly. Each pen contains 4 doses (4 weeks supply). Wegovy starts at 0.25 mg and escalates to a maximum of 2.4 mg once weekly, also using a pre-filled auto-injection pen.
Both medications are injected once weekly into the stomach, thigh, or upper arm. Detailed injection instructions are provided in the patient information leaflet (PIL) accompanying each prescription. Your prescriber or pharmacist will also provide guidance on injection technique, site rotation, and storage. Storage differs slightly: Mounjaro unused pens are refrigerated (2–8°C); Wegovy follows similar refrigeration guidelines. After first use, both can be stored at room temperature for limited periods.
- 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.
What's the main difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, activating two hormone pathways. Wegovy (semaglutide) is a GLP-1 agonist only. The dual mechanism in Mounjaro may provide additional appetite regulation, though both medications support weight loss alongside lifestyle changes.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications must be managed by your prescriber. There is no official conversion chart between Mounjaro and Wegovy doses. Your prescriber will assess whether a switch is appropriate based on your individual response and health profile.
Which injection is easier to use, Mounjaro or Wegovy?
Both use pre-filled pens designed for ease of use. Mounjaro comes as a KwikPen with 4 doses per device; Wegovy is a pre-filled auto-injection pen. Both are administered once weekly. Your prescriber or pharmacist will demonstrate injection technique—ease of use is often a matter of personal preference.
Do Mounjaro and Wegovy have the same side effects?
Both commonly cause nausea, diarrhoea, constipation, and stomach pain, especially during dose escalation. Serious side effects like pancreatitis are rare but require immediate medical attention. Side effect profiles are very similar, though individual experiences vary. Your prescriber will discuss potential side effects before you start.
How do I get Mounjaro or Wegovy instructions in the UK?
Both medications come with a patient information leaflet (PIL) in the prescription pack. You'll also receive injection instructions from your prescriber or pharmacist. Use our free GLP-1 weight loss calculator to understand potential outcomes, and always follow your prescriber's specific guidance for your dose and timeline.
Ready to take control of your weight loss journey?
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