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 Syringes vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) come in pre-filled syringe forms, but they work differently and have distinct dosing schedules. This guide explains the practical differences between the two, including their mechanisms, delivery systems, and how they're prescribed in the UK. Understanding the distinction helps you have a more informed conversation with your GP or prescriber about which option might be more suitable for your individual circumstances.
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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 Differ in Mechanism and Delivery
Mounjaro uses a KwikPen pre-filled injection device and contains tirzepatide, a dual GIP and GLP-1 receptor agonist. This dual mechanism activates two hormone pathways simultaneously, which is why Mounjaro may differ in effect from single-mechanism medications. Mounjaro starts at 2.5 mg weekly and titrates up to a maximum of 15 mg weekly, with minimum four-week intervals between dose increases.
Wegovy comes as a pre-filled auto-injection pen and contains semaglutide, a GLP-1 receptor agonist only. It activates a single hormone pathway. Wegovy begins at 0.25 mg and escalates to 2.4 mg maximum over approximately 16–20 weeks. Both are once-weekly injections prescribed following a clinical assessment in the UK.
- Mounjaro: dual mechanism (GIP + GLP-1), 2.5–15 mg weekly
- Wegovy: single mechanism (GLP-1 only), 0.25–2.4 mg weekly
- Both require UK prescription and clinical eligibility check
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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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Clinical Trial Data and Individual Suitability
Clinical trial evidence informs prescriber decisions, though individual results vary significantly. In the SURMOUNT-1 trial, Mounjaro at 15 mg achieved average weight loss of approximately 20.9% body weight over 72 weeks. The STEP-1 trial showed Wegovy 2.4 mg produced average weight loss of approximately 14.9% over 68 weeks. Both trials included reduced-calorie diet and increased physical activity. Your GP or licensed prescriber will assess your medical history, current medications, and weight-related health conditions to determine which medication—if either—is appropriate for you individually. Neither medication is suitable for everyone.
Practical considerations also matter. Mounjaro KwikPen requires once-weekly injection at a higher starting dose. Wegovy's gradual titration may suit those sensitive to side effects. Both require careful storage (refrigerated before use), and both may cause nausea, diarrhoea, or constipation initially. Use our free GLP-1 weight loss calculator to see potential projected outcomes at different doses, and discuss findings with your prescriber. Your eligibility, preference, and medical profile ultimately determine which is prescribed.
- 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 is the "golden dose" of Mounjaro, and does Wegovy have one?
The term "golden dose" refers informally to the maximum therapeutic dose where most benefit is achieved—for Mounjaro this is typically 15 mg weekly, for Wegovy it's 2.4 mg weekly. Neither has an officially designated "golden dose"; the right dose is individualised based on response and tolerability. Your prescriber will determine the optimal dose for you.
Can I switch from Mounjaro to Wegovy or vice versa in the UK?
Switching between Mounjaro and Wegovy is possible but must be managed by your prescriber. There is no official conversion chart, though approximate equivalences exist (for example, Mounjaro 15 mg is roughly comparable to Wegovy 2.4 mg). Any switch requires medical supervision and typically involves stopping one medication and starting the other on a new titration schedule.
Which syringe/pen is easier to use, Mounjaro KwikPen or Wegovy auto-injector?
Both are pre-filled single-use devices designed for ease of use. Mounjaro KwikPen is reusable for four weekly doses (one KwikPen = 4 weeks) before disposal. Wegovy pens are single-dose. Preference depends on individual comfort with injection devices. Your prescriber or pharmacist can demonstrate proper injection technique.
Are Mounjaro and Wegovy available on the NHS in the UK?
Both are licensed in the UK for weight management. NHS availability is limited to specialist tier 3/4 weight management services, and access varies significantly by region and waiting lists. Private UK prescribing through online clinics is more readily available. Discuss NHS versus private options with your GP.
What should I do if I experience side effects with one medication but not the other?
Side effect profiles are similar between Mounjaro and Wegovy (nausea, diarrhoea, constipation common), though individual responses vary. If side effects are intolerable, contact your prescriber before stopping. Do not switch medications without guidance. Your prescriber may adjust your dose, change the medication, or recommend lifestyle support. Never self-adjust or discontinue without advice.
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