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 vs Wegovy: Side Effects Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weekly weight loss injections licensed in the UK, but they work differently and may cause different side effects. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 receptor agonist only. Understanding how each medication affects the body can help you and your prescriber decide which is more suitable for your individual health needs. This guide compares the side effects, mechanisms, and clinical trial data for both medications.
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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
The key difference between Mounjaro and Wegovy lies in their mechanism of action. Wegovy (semaglutide) activates only the GLP-1 receptor, which signals fullness to the brain, slows stomach emptying, and helps regulate blood sugar. Mounjaro (tirzepatide) activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 receptors — a dual mechanism that may provide additional appetite suppression.
In clinical trials, Mounjaro users lost an average of 15–21% of body weight on the highest dose, whilst Wegovy users lost approximately 15% on average. Both medications are taken as a once-weekly injection, though Wegovy takes longer to reach maximum dose (17 weeks vs 20 weeks for Mounjaro). Because of these differences, side effects can vary between the two, though both share common gastrointestinal effects like nausea and diarrhoea.
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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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Side Effects: What's Similar and What Differs
Common to both medications: nausea, diarrhoea, constipation, vomiting, and stomach pain are the most frequently reported side effects with both Mounjaro and Wegovy. These typically improve over time as the body adjusts.
Potentially more common with Mounjaro: Some users report slightly higher rates of nausea and stomach discomfort, possibly due to the dual GIP/GLP-1 mechanism. Hair loss (alopecia) has been reported more frequently in Mounjaro trials than Wegovy, though this remains uncommon.
Serious side effects like pancreatitis, gallbladder problems, or severe allergic reactions are rare with both medications. Any decision between them should be made with your prescriber following a clinical assessment of your individual health, medical history, and how you've tolerated similar medications previously.
- 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 Mounjaro or Wegovy more likely to cause side effects?
Both medications cause similar overall rates of side effects. Mounjaro may cause slightly more nausea and stomach discomfort due to its dual mechanism, whilst hair loss has been reported more frequently with Mounjaro. However, individual responses vary significantly — some people tolerate one far better than the other. Your prescriber will consider your personal history when choosing.
Do side effects improve over time on both medications?
Yes. Most side effects with both Mounjaro and Wegovy improve within the first 4–8 weeks as your body adjusts. Nausea and digestive changes typically become less bothersome over time. If side effects persist or worsen, speak to your prescriber — dose adjustments or switching medications may be options.
Can you switch from Wegovy to Mounjaro if you don't tolerate the side effects?
Potentially, yes. If Wegovy causes side effects you cannot tolerate, your prescriber may try Mounjaro instead. There is no official conversion chart, but approximate equivalences exist (e.g., Wegovy 1.7 mg ≈ Mounjaro 10 mg). Any switch must be managed by your prescriber with appropriate dose adjustments.
Which injection has worse side effects — Mounjaro or Wegovy?
Neither has "worse" side effects overall — they have similar profiles. Mounjaro may cause slightly more nausea; Wegovy has a longer titration schedule. Individual tolerance varies dramatically. The "better" choice depends entirely on your personal health, medical history, and how your body responds. This is why clinical assessment is essential.
Should I use the BMI calculator to decide which medication is right for me?
The BMI calculator can help you understand your starting point and eligibility range, but it cannot determine which medication is suitable for you. Both Mounjaro and Wegovy require a full clinical assessment by a UK-licensed prescriber, who will review your medical history, existing medications, and individual risk factors before recommending one over the other.
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