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: Common Side Effects Compared
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections used in the UK, but they work differently and may cause different side effects. Understanding these differences can help you and your GP make an informed decision about which medication might be suitable for you. This guide compares the common side effects of both medications based on clinical trial data and real-world use in the UK.
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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.
Clinical Trial Data: Side Effect Profiles
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, approved by the MHRA for weight management in UK adults. In the SURMOUNT-1 clinical trial, the most common side effects were gastrointestinal: nausea (25% of participants), diarrhoea (22%), vomiting (6%), and constipation (21%). These typically occurred during dose titration and often improved over time.
Wegovy (semaglutide) is a single GLP-1 receptor agonist. In the STEP-1 clinical trial, the side effect profile was similar but with slightly lower rates in some categories: nausea (25%), diarrhoea (23%), vomiting (7%), and constipation (12%). Both medications showed gastrointestinal side effects as the primary concern, though individual experiences vary considerably.
The dual-action mechanism of Mounjaro may explain slightly higher rates of nausea and diarrhoea in some users, though clinical evidence shows both are well-tolerated for most people when dose escalation is gradual. Side effects typically decrease after the first few weeks of treatment.
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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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Which Medication Might Suit You?
Both Mounjaro and Wegovy require a prescription and clinical assessment before use. Your GP or prescriber will consider your medical history, current medications, and personal tolerance for side effects when deciding between them. Neither medication is universally "better" — the right choice depends on your individual circumstances.
If you've experienced significant side effects with one GLP-1 medication previously, your prescriber may suggest trying the other. Some people tolerate the gastrointestinal effects better than others, and there's often a period of adjustment. Using our free GLP-1 weight loss calculator can help you understand the potential benefits of either medication, but the decision about which is appropriate for you must be made in consultation with your prescriber.
Q1: What are the most common side effects of Mounjaro? A1: The most common side effects of Mounjaro are gastrointestinal: nausea, diarrhoea, vomiting, and constipation. These typically occur during the first few weeks and often improve as your body adjusts. Nausea affected around 25% of trial participants, with diarrhoea at 22%. Side effects usually decrease after the initial adjustment period. Q2: How do Wegovy side effects differ from Mounjaro? A2: Wegovy and Mounjaro cause similar types of side effects (mainly gastrointestinal), but rates vary slightly. Wegovy showed slightly lower constipation rates (12% vs 21%) in clinical trials. Both are well-tolerated for most people. The difference in mechanism (Wegovy is GLP-1 only; Mounjaro is dual GIP+GLP-1) may account for minor variations in individual experience. Q3: How long do side effects last with these injections? A3: Most side effects are temporary and typically improve within the first 4–8 weeks as your body adjusts to the medication. Gastrointestinal symptoms are most common during the initial dose titration phase. If side effects persist beyond 8 weeks, discuss this with your GP, as adjustments may help. Q4: Can I switch from Mounjaro to Wegovy if side effects are too much? A4: Yes, switching between medications is possible and is sometimes recommended if side effects are unmanageable. This must be arranged through your prescriber, who will manage the transition carefully. The approximate dose equivalence is Mounjaro 5mg ≈ Wegovy 0.5mg, though any switch requires professional guidance. Q5: Which injection causes fewer side effects in the UK? A5: Clinical trials suggest side effect rates are comparable between Mounjaro and Wegovy. Neither is definitively "easier to tolerate" — individual responses vary significantly. What matters is finding the medication that works best for your body. Your prescriber will help you decide based on your medical history and tolerance level.- 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 is BMI and how is it calculated?
BMI (Body Mass Index) is calculated by dividing your weight in kilograms by your height in metres squared. It's the standard screening tool used by the NHS to assess whether adults are a healthy weight.
Is BMI accurate for everyone?
BMI is a useful screening tool but doesn't account for muscle mass, bone density, or body fat distribution. Athletes may have a high BMI without excess fat. The NHS recommends using BMI alongside waist circumference for a more complete picture.
What are GLP-1 medications like Mounjaro and Wegovy?
GLP-1 receptor agonists are prescription medications that mimic gut hormones to reduce appetite and help with weight loss. Mounjaro (tirzepatide) and Wegovy (semaglutide) are the two most widely used for weight management in the UK.
Am I eligible for GLP-1 weight loss medication in the UK?
Generally, you need a BMI of 30+ (obese) or 27+ with a weight-related health condition. You must be 18 or over. Use our BMI calculator to check.
What is the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist with up to 22.5% weight loss. Wegovy (semaglutide) is GLP-1-only with up to 15-17% weight loss. Both are weekly injections.
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