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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Downsides of Mounjaro — What to Know
Mounjaro (tirzepatide) is a prescription-only weight management injection that works differently to other GLP-1 medications like Wegovy, and both have potential downsides to consider. While clinical trials showed significant average weight loss, side effects, cost, accessibility, and individual suitability vary considerably. This guide compares the practical drawbacks of Mounjaro versus Wegovy to help you understand what matters for your situation. Both medications require a clinical assessment before prescription, and your GP or prescriber will help determine which—if either—is right for you.
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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 Data and Side Effect Profiles
The SURMOUNT clinical trial showed Mounjaro helped participants lose an average of 15–21% body weight, but side effects were common. Nausea, diarrhoea, vomiting, and constipation affected more than 1 in 10 users, particularly during dose titration when escalating from 2.5mg to higher doses. In comparison, Wegovy's STEP trials demonstrated approximately 15% average weight loss with a similar side effect profile, though the single GLP-1 mechanism differs from Mounjaro's dual GIP+GLP-1 approach. Individual side effect severity varies greatly—some users experience mild, temporary effects while others find them limiting enough to stop treatment. Hair loss (alopecia) was reported in clinical trials, though less commonly discussed. The key downside: side effects are unpredictable and may persist for weeks even at maintenance doses. Your GP can discuss how side effects compare between options using our free GLP-1 weight loss calculator to project potential outcomes.
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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
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Practical Downsides and Suitability Factors
Accessibility and cost remain significant downsides. NHS access to Mounjaro through specialist weight management services involves waiting lists that can stretch months or years depending on your region's integrated care board (ICB). Private prescriptions cost £129–£320/month depending on dose, and you must obtain a new prescription every 4 weeks or at each dose change. Storage requires refrigeration at 2–8°C, and unopened pens must be disposed of after 30 days of room-temperature storage—adding waste and cost concerns. Mounjaro's KwikPen device requires a UK pharmacy injection technique, and missing doses by more than 4 days means waiting until your next scheduled injection. For some users, the commitment required and ongoing expense create a practical downside not present with lifestyle-only approaches. A prescriber will assess your individual circumstances—medical history, other medications, and personal priorities—to determine suitability. Use our free BMI calculator to establish your baseline and discuss your health context with your GP.
- 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 downsides of Mounjaro?
The most frequent downsides are gastrointestinal side effects—nausea, diarrhoea, vomiting, and constipation affect more than 1 in 10 users. Dose titration (gradually increasing from 2.5mg to higher doses) can mean 4–5 weeks of adjustment discomfort. Other downsides include injection site reactions, fatigue, hair loss in some users, and the need for refrigerated storage. Individual experience varies considerably.
Is Mounjaro safer than Wegovy?
Both medications have similar safety profiles and side effect rates according to clinical trials. Mounjaro uses a dual GIP+GLP-1 mechanism versus Wegovy's single GLP-1 action, but neither is inherently "safer"—suitability depends on individual health factors. Your prescriber will assess which is more appropriate for your medical history and other medications.
How does Mounjaro cost compare to Wegovy as a downside?
Both carry similar private costs (£129–£320/month depending on dose). Neither is available on NHS prescription for weight management as standard—access requires referral to specialist tier 3 services with long waiting lists in many areas. The monthly cost and ongoing prescription requirement are significant practical downsides for both medications.
Can I switch from Mounjaro to Wegovy if I experience downsides?
Yes, switching is possible but must be managed by your prescriber. There is no official conversion chart—approximate equivalence exists (e.g., Mounjaro 10mg is roughly equivalent to Wegovy 1.7mg), but any change requires clinical guidance. Some side effects improve over time, so your prescriber may adjust your dose rather than switch medications.
What happens if Mounjaro's downsides outweigh the benefits?
If side effects are intolerable or weight loss is insufficient, stopping is an option—discuss this with your prescriber. Stopping suddenly is safe, but weight regain often occurs within weeks. Some users find lifestyle changes, the NHS Weight Loss Plan, or community weight management programmes equally or more sustainable long-term than medication.
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.