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 weight management injections licensed in the UK, but they work differently and have distinct side effect profiles. Understanding these differences helps you and your prescriber decide which medication may be more suitable for your individual circumstances. This guide compares the side effects, mechanisms, and trial data for both medications to help you make an informed decision about weight management options.
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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 simultaneously. This dual mechanism may enhance appetite regulation and support a greater sense of fullness after meals. Wegovy (semaglutide) is a GLP-1-only receptor agonist, activating a single hormone pathway. In clinical trials, the SURMOUNT-1 study showed Mounjaro helped participants lose an average of 20.9% of body weight at the maximum 15mg dose, whilst the STEP-1 trial showed Wegovy achieved approximately 14.9% average weight loss at 2.4mg. Both medications slow gastric emptying and reduce appetite, but the dual mechanism of Mounjaro may account for slightly higher average weight loss in trials. Individual results vary significantly, and both are prescribed alongside reduced-calorie diet and increased physical activity.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Side Effects: Frequency and Severity
Both medications share a similar side effect profile since they target appetite-related pathways. Very common side effects (affecting more than 1 in 10 people) for both include nausea, diarrhoea, vomiting, constipation, and stomach pain. These typically improve as your body adjusts to the medication. Common side effects include indigestion, fatigue, dizziness, and injection site reactions. Mounjaro users report slightly higher rates of nausea during dose titration, whilst Wegovy users sometimes report more persistent constipation. Neither medication is right or wrong—individual tolerance varies. Your GP or prescriber can discuss which medication's side effect profile may suit you better based on your medical history and preferences. Use our free GLP-1 weight loss calculator to explore projected outcomes with either medication.
- 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 are the most common side effects of Mounjaro?
Very common side effects of Mounjaro include nausea, diarrhoea, vomiting, constipation, and stomach pain. These usually improve within the first few weeks as your body adjusts. Common side effects include indigestion, fatigue, dizziness, headache, and injection site redness or bruising. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and allergic reactions—seek immediate medical attention if these occur.
How do Wegovy side effects compare to Mounjaro?
Wegovy has a nearly identical side effect profile to Mounjaro because both are GLP-1 medications. The main difference is mechanism: Mounjaro's dual GIP+GLP-1 action may cause slightly more nausea during titration, whilst Wegovy users sometimes experience more prolonged constipation. Both cause gastrointestinal effects primarily. Individual experience varies—some tolerate Wegovy better and vice versa. Your prescriber can help decide based on your health history.
Which medication has fewer side effects, Mounjaro or Wegovy?
Clinical trials and real-world data show both have similar side effect rates and types. Neither has "fewer" side effects overall—the difference is individual tolerance. Some people tolerate Mounjaro better, others prefer Wegovy. Side effect severity often depends on dose titration speed, diet quality, and individual metabolism rather than the medication itself. Your prescriber will start at the lowest dose and increase gradually to minimise side effects.
Do side effects go away with Mounjaro and Wegovy?
Yes, for most people. Common side effects like nausea, diarrhoea, and constipation typically improve within 2–4 weeks as your body adjusts to the medication. Staying hydrated, eating smaller portions, and avoiding fatty foods can help manage gastrointestinal side effects. If side effects persist beyond 4–6 weeks, discuss this with your GP—dose adjustment or switching medications may help.
Can I switch from Mounjaro to Wegovy if I get side effects?
Potentially, yes. If you experience troublesome side effects on one medication, your prescriber may consider switching to the other. However, switching requires a medical assessment and may involve a wash-out period. It's not a simple switch—Mounjaro 10mg is approximately equivalent to Wegovy 1.7mg, but any medication change must be managed by your prescriber. Always discuss side effects with your GP before considering a change.
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