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 Dos and Don'Ts: Comparison with Wegovy
If you're considering tirzepatide (Mounjaro) or semaglutide (Wegovy) for weight management in the UK, understanding what to do — and what to avoid — with each medication is essential. Both are prescription-only GLP-1-based weight loss injections, but they work slightly differently, have different dose ranges, and may suit different individuals. This guide compares the dos and don'ts of each to help you understand what a prescriber might recommend based on your health profile. Remember, only a UK-licensed prescriber can assess which medication is right for you.
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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 help regulate appetite more powerfully than a single-pathway medication. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating one hormone pathway. Clinical trial data shows both are effective: the SURMOUNT trial showed Mounjaro users lost an average of 15–21% of body weight, while the STEP-1 trial showed Wegovy users lost approximately 15% body weight — though individual results vary significantly.
- Do understand that Mounjaro's dual action may offer a different weight loss profile
- Do know that Wegovy has been used for longer, with more long-term safety data in the UK
- Don't assume one is automatically "better" — suitability depends on your individual health and response
- Don't switch between medications without prescriber guidance
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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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Dos and Don'Ts for Dosing and Administration
Both medications are once-weekly injections delivered via pre-filled KwikPen devices. Mounjaro dose range: 2.5 mg (starting) up to 15 mg maximum. Wegovy dose range: 0.25 mg (starting) up to 2.4 mg maximum. Do inject on the same day each week and rotate injection sites (stomach, thigh, upper arm). Do store unopened pens in a fridge at 2–8°C. Do allow 4 weeks minimum at each dose before your prescriber considers increasing. Don't miss doses by more than 4 days — if you do, skip that dose and take the next scheduled injection. Don't take a double dose or freeze pens. Use our free GLP-1 weight loss calculator to understand projected outcomes at different doses.
- 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 (tirzepatide) is a dual GIP+GLP-1 agonist activating two hormone pathways, while Wegovy (semaglutide) is a GLP-1 agonist only. Both are once-weekly injections licensed in the UK for weight management. Mounjaro's dual mechanism may offer different appetite regulation. Both require a UK-licensed prescriber's clinical assessment before prescription. Individual response varies.
Which medication should I choose — Mounjaro or Wegovy?
Only a prescriber can decide which is suitable based on your health profile, BMI, weight-related conditions, and medical history. Both are effective, but suitability depends on individual factors. If you're considering GLP-1 treatment, ask your GP or online prescriber which medication they recommend for your circumstances. Use our BMI calculator to understand your starting health metrics.
What are the dos and don'ts for side effects?
Do expect nausea, diarrhoea, constipation, and stomach pain — these are very common, especially when starting. Do stay hydrated and eat small, frequent meals. Don't ignore severe, persistent stomach pain — contact your GP urgently as this could indicate pancreatitis. Don't expect side effects to last forever; many ease after a few weeks as your body adjusts.
Do I need a prescription to use Mounjaro or Wegovy in the UK?
Yes, both are prescription-only medicines. You cannot buy them over the counter. Access is through NHS specialist weight management services or private online prescribers following a clinical assessment. You'll need to be assessed for eligibility based on BMI (usually 30+, or 27+ with weight-related conditions) and medical history.
Can I switch from Mounjaro to Wegovy or vice versa?
Any switch between medications must be managed by your prescriber — there's no standard conversion chart. Approximate equivalence exists (Mounjaro 5 mg ≈ Wegovy 0.5 mg), but switching requires medical supervision. Your prescriber will assess whether a change is appropriate and how to transition safely without losing efficacy.
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