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
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only weight management injections available in the UK, but they work differently and can cause different side effects. At the 5mg dose, Mounjaro is in the early escalation phase, whilst Wegovy at equivalent strength is typically at a mid-range dose. Both medications are GLP-1 receptor agonists—though Mounjaro is technically a dual GIP and GLP-1 agonist—meaning side effects often overlap, but frequency and severity can vary between individuals. Understanding these differences helps you and your prescriber decide which option might suit you best.
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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 5mg and Wegovy Work Differently
The key difference between these medications affects how side effects develop. Mounjaro (tirzepatide) activates two hormone pathways: GIP and GLP-1, whilst Wegovy (semaglutide) activates GLP-1 only. This dual mechanism in Mounjaro may explain why some users report slightly different nausea patterns or appetite suppression timing compared to Wegovy.
In clinical trials, the SURMOUNT-1 trial (Mounjaro) and STEP-1 trial (Wegovy) both reported similar common side effects: nausea, diarrhoea, vomiting, and constipation. However, individual experiences vary significantly. At 5mg, Mounjaro is typically a starting or early escalation dose, whilst Wegovy users at equivalent strength have usually progressed further through dose escalation. This timing difference may mean side effect profiles feel different even if the underlying mechanisms are similar.
Both medications slow gastric emptying (how quickly food leaves your stomach), which is why gastrointestinal side effects are most common early in treatment and often improve over time as your body adjusts.
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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.
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Which Medication Causes Different Side Effects?
Side effect frequency and severity depend more on individual physiology than the medication itself. Some users tolerate Mounjaro 5mg better; others find Wegovy suits them more. Neither medication is inherently "safer" than the other—clinical trial data shows comparable safety profiles overall.
Factors affecting your personal side effects include: your baseline digestive health, other medications you take, how quickly you escalate doses, and your individual metabolism. Your prescriber will monitor you closely and can adjust your dose or switch medications if side effects become bothersome. Both are prescription-only and require a clinical assessment before starting, ensuring your individual health context is considered.
If you're deciding between these options, use our free GLP-1 weight loss calculator to explore projected outcomes, then discuss side effect tolerability with your prescriber—they'll help choose the best fit for you.
- 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 5mg safer than Wegovy for side effects?
No. Clinical trials show Mounjaro and Wegovy have comparable safety profiles. Side effect experience is highly individual. Both medications cause similar common side effects (nausea, diarrhoea, constipation), and neither is inherently safer. Your prescriber will assess which suits your personal health context best.
Does Mounjaro 5mg cause worse nausea than Wegovy?
Not necessarily. Nausea is very common with both medications. The dual GIP+GLP-1 mechanism of Mounjaro may cause slightly different appetite suppression timing compared to Wegovy's GLP-1-only action, but severity varies widely between individuals. Most users report nausea improves as the body adjusts.
What are the most common side effects at 5mg for both medications?
Both commonly cause nausea, diarrhoea, vomiting, constipation, and stomach pain—especially in the first few weeks. These are listed as "very common" in clinical data. Side effects often improve over time as your body adapts to the medication.
Can I switch from Mounjaro 5mg to Wegovy if side effects are bad?
Yes, switching between GLP-1 medications is possible and managed by your prescriber. However, allow at least 2–4 weeks at a dose to see if side effects settle naturally before switching. Your prescriber will decide the safest switching approach.
How do I know which medication will suit me better?
Only a clinical assessment can determine suitability. Your prescriber considers your medical history, other medications, digestive health, and weight management goals. Both are prescription-only and require this assessment before starting. Neither can be predicted to suit you without professional review.
Ready to take control of your weight loss journey?
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