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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5 mg Mounjaro Side Effects vs Wegovy: A Factual Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections available in the UK, but they work differently and may cause different side effects. At 5 mg, Mounjaro is typically a mid-range dose during the titration phase, whilst Wegovy at 0.5 mg is an early escalation dose. This guide compares side effects from clinical trial data, explains how their mechanisms differ, and helps you understand what prescribers consider when choosing between them. Use our free GLP-1 weight loss calculator to explore potential outcomes with either medication.
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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/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.
Clinical Trial Side Effects: SURMOUNT vs STEP Data
Mounjaro (tirzepatide) was studied in the SURMOUNT clinical trial. At the 5 mg dose during escalation, common side effects included nausea (25–30%), diarrhoea (20%), vomiting (9–11%), constipation (16–20%), and stomach pain (18–24%). These often improve as the body adjusts over weeks 1–4 at each dose level.
Wegovy (semaglutide) was evaluated in the STEP trials. At 0.5 mg (equivalent escalation phase), nausea affected 26–34% of participants, diarrhoea 20–23%, vomiting 5–7%, and constipation 16–20%. Injection site reactions (redness, bruising) occurred in roughly 2–5% of users.
Both medications show that gastrointestinal side effects are most common early in treatment and often decrease with continued use and dose adaptation. Individual experiences vary significantly based on diet, hydration, and personal tolerance.
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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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How Mounjaro and Wegovy Differ: Mechanism and Dose Context
The key difference is mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates only the GLP-1pathway. At 5 mg, Mounjaro's dual action may cause stronger early appetite suppression, which can lead to more pronounced nausea during titration. At 0.5 mg, Wegovy is still in its early escalation phase with a gentler effect profile.
There is no direct dose equivalence between them. A UK-licensed prescriber assesses your individual health, medical history, and previous medication tolerance to decide which is more suitable. Both require a clinical assessment and prescription. Track your progress with our free progress tracker once you begin treatment, and always report persistent side effects to your prescriber—dose adjustments or a slower titration can help.
- 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.
Is nausea worse on 5 mg Mounjaro than Wegovy?
Clinical trial data suggests nausea rates are similar (25–30% on Mounjaro 5 mg vs 26–34% on Wegovy 0.5 mg). Individual responses vary. Some people tolerate one better than the other. Your prescriber can assess which is likely better for you based on your health history and previous medication experiences.
How long do side effects last at 5 mg Mounjaro?
Most gastrointestinal side effects peak in the first 1–2 weeks at a new dose and typically improve by weeks 3–4 as your body adjusts. However, persistent side effects should be reported to your prescriber. They may slow your titration, adjust your diet approach, or consider switching medications if needed.
Can I switch from Wegovy to Mounjaro if side effects are too strong?
Yes, switching is possible and should be managed by your prescriber. There is no official conversion chart, but approximate equivalences guide the switch (0.5 mg Wegovy ≈ 5 mg Mounjaro). Your prescriber will assess whether a switch is appropriate and how to transition safely without a dangerous gap.
What makes Mounjaro's side effects different from Wegovy's?
Mounjaro activates two hormone pathways (GIP and GLP-1), whilst Wegovy activates only GLP-1. This dual mechanism may cause slightly stronger early appetite suppression and gastric effects. However, clinical trial side effect profiles are remarkably similar, and individual variation is the largest factor in how you experience either medication.
Should I avoid food when starting 5 mg Mounjaro to manage nausea?
No—eating small, frequent, bland meals (porridge, toast, soup) often helps. Skipping meals can worsen nausea and energy levels. Stay hydrated and avoid fatty or spicy foods during the first weeks. Your GP or prescriber can advise on nutrition strategies specific to your situation.
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