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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Taking Mounjaro For The First Time: Mounjaro vs Wegovy
Starting tirzepatide (Mounjaro) for the first time can feel daunting. If you're comparing it with Wegovy, understanding the key differences will help you and your prescriber make an informed choice. Both are prescription-only GLP-1-based weight management injections available in the UK, but they work slightly differently and may suit different people. This guide compares both options to help you understand what to expect when beginning treatment.
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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 in your body. This dual mechanism may enhance appetite regulation and support fuller feelings after meals. Wegovy (semaglutide) is a GLP-1-only agonist, activating a single pathway.
Clinical trial data shows the difference in outcomes: in the SURMOUNT-1 trial, Mounjaro at maximum dose (15 mg) achieved average weight loss of approximately 20.9% of body weight over 72 weeks. In the STEP-1 trial, Wegovy (2.4 mg) achieved average weight loss of approximately 14.9% over 68 weeks. Both results were alongside a reduced-calorie diet and increased physical activity. Individual results vary significantly, and these are averages from clinical populations.
Your prescriber will assess which mechanism suits your health profile. Both require a Mounjaro KwikPen prescription or Wegovy auto-injector, depending on which is recommended for you. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses.
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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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Starting Treatment: Dose Escalation and Suitability
When taking Mounjaro for the first time, you'll begin on a low dose (2.5 mg weekly) and gradually increase every 4 weeks until reaching your target maintenance dose—typically 5 mg, 10 mg, or 15 mg. Wegovy starts at 0.25 mg and escalates over 16+ weeks to a maximum of 2.4 mg. The slower Mounjaro escalation means reaching effective doses faster, whilst Wegovy's gradual approach may reduce initial side effects.
Both medications are prescription-only and require clinical assessment by a UK-licensed prescriber. Your GP, specialist weight management service (on the NHS), or private online prescriber will evaluate your BMI, health conditions, and medication history to determine suitability. Neither is suitable during pregnancy or for under-18s. Side effects are similar for both—nausea, diarrhoea, and constipation are common, especially in the first weeks. Your prescriber will discuss which option aligns with your health profile and 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 → 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?
Mounjaro is a dual GIP and GLP-1 agonist (activates two hormone pathways), whilst Wegovy is GLP-1 only. This means Mounjaro may work through two mechanisms, which some clinical evidence suggests could offer greater weight loss support on average. However, individual results vary, and the best choice depends on your personal health profile and prescriber assessment.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications is possible but must be managed by your prescriber. There is no official established conversion chart—your prescriber will assess your current response and recommend a starting dose on the new medication. Never switch without medical guidance.
Which causes fewer side effects—Mounjaro or Wegovy?
Side effects are similar for both: nausea, diarrhoea, and constipation are common early on. Mounjaro's faster dose escalation means some people reach therapeutic doses quicker, whilst Wegovy's slower escalation may allow more gradual adjustment. Individual tolerance varies significantly. Both side effects typically improve after the first few weeks.
How quickly will I lose weight on Mounjaro compared to Wegovy?
Clinical trials show Mounjaro achieved average weight loss of 20.9% at maximum dose, whilst Wegovy achieved 14.9%. However, these are trial averages with lifestyle support—individual results vary greatly based on genetics, adherence, diet, and activity. Your prescriber can discuss realistic expectations for you personally.
Do I need a Mounjaro KwikPen prescription from my GP?
Mounjaro is prescription-only and requires clinical assessment. You can access it through NHS specialist tier 3 weight management services (referral from GP, but availability varies by region) or through a private online prescriber following an assessment. Either way, a UK-licensed prescriber must issue your prescription.
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