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 5mg Side Effects vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, but they work slightly differently and may cause different side effect profiles. At the 5mg Mounjaro dose — typically used during the dose escalation phase — the side effect pattern is broadly similar to Wegovy, though individual experiences vary significantly. Understanding how these medications compare can help you and your prescriber decide which might be more suitable for your circumstances. This guide compares the side effects of both medications using NHS-aligned information and clinical trial data.
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Calculate Your BMI Instantly
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: Mounjaro vs Wegovy
Mounjaro (tirzepatide) at 5mg represents the first maintenance dose in the SURMOUNT-1 clinical trial. Common side effects reported during this phase included nausea (25–30% of participants), diarrhoea (22%), vomiting (7–9%), and constipation (16–21%). These typically emerge during the dose escalation phase and often improve as the body adjusts over weeks 4–8.
Wegovy (semaglutide) at equivalent doses (0.5mg during early escalation) showed similar frequency in STEP-1 trials: nausea (25%), diarrhoea (21%), vomiting (8%), and constipation (16%). The key difference is mechanism — Mounjaro is a dual GIP and GLP-1 agonist, whilst Wegovy is GLP-1-only. This dual action in Mounjaro may theoretically cause slightly different gastrointestinal responses, though clinical evidence shows comparable overall tolerability at early doses.
- Nausea: more common in first 2–4 weeks, usually improves
- Gastrointestinal upset: diarrhoea and constipation both reported — individual variation
- Injection site reactions: redness or bruising at injection point (both medications)
- Fatigue and headache: reported in 5–10% of users with both medications
Most side effects peak during dose escalation and reduce substantially by week 8–12 on a stable dose.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Which Medication Might Suit You Better?
The choice between Mounjaro and Wegovy depends on individual health circumstances, not just side effect profiles. Both require a prescription and clinical assessment — your GP or licensed prescriber will evaluate whether you're eligible based on your BMI, medical history, and any weight-related health conditions. Some people tolerate one medication better than the other, but this cannot be predicted in advance.
Prescriber considerations: Your prescriber may recommend Mounjaro if you have type 2 diabetes or prediabetes (the dual mechanism may benefit glucose control), or Wegovy if you prefer a single-mechanism medication or have specific contraindications. Both are UK-licensed for adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions.
Try our GLP-1 weight loss calculator to understand potential weight loss projections at different doses, and use our BMI calculator to check your current status. Individual tolerance varies — side effects at 5mg Mounjaro are typically mild and often temporary.
- 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.
Are the side effects of Mounjaro 5mg the same as Wegovy?
Side effects are broadly similar — both cause nausea, diarrhoea, and constipation in roughly 20–30% of users during early doses. The frequency and severity vary by individual. Mounjaro's dual GIP+GLP-1 mechanism may cause slightly different gastrointestinal timing compared to Wegovy's GLP-1-only approach, but clinical trials show comparable overall tolerability at equivalent dose stages.
Do Mounjaro 5mg side effects get better over time?
Yes. Most users experience peak side effects during the first 2–4 weeks on a new dose, with significant improvement by week 8–12 as the body adjusts. Gastrointestinal symptoms like nausea and constipation often reduce substantially once you stabilise on 5mg. Always report persistent side effects to your prescriber.
Which medication has fewer side effects — Mounjaro or Wegovy?
Clinical trial data shows comparable side effect frequencies. SURMOUNT trials (Mounjaro) and STEP trials (Wegovy) report nausea in approximately 25% of participants at early doses. Individual tolerance varies significantly — some people tolerate one better than the other, but this cannot be predicted before starting.
Can I switch from Mounjaro 5mg to Wegovy if I get side effects?
Possibly, but any medication change must be managed by your prescriber. Some side effects improve with time on the same dose rather than switching. Your GP or licensed prescriber can discuss alternatives if Mounjaro 5mg is not tolerated, which may include staying at a lower dose, switching medications, or pausing treatment.
Is Mounjaro 5mg or Wegovy available on the NHS in the UK?
Both are available through NHS specialist (tier 3) weight management services in some regions, though access varies significantly by area and waiting lists can be lengthy. Both are also available privately through GPhC-registered online pharmacies following a clinical assessment. Availability and cost depend on your route to prescription.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.