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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Is Monjaro Safe? Monjaro vs Wegovy Safety Comparison
Both Monjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections approved in the UK and have strong safety profiles based on clinical trial data. However, they work through different mechanisms — Monjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 receptor agonist only. Both carry similar common side effects, but individual suitability depends on your personal health profile and should be assessed by a prescriber. This guide compares the safety evidence for both medications to help you understand the differences.
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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.
Safety Profiles: SURMOUNT vs STEP Clinical Trial Data
Monjaro safety comes from the SURMOUNT clinical trial programme, which tested tirzepatide in over 13,000 participants. Wegovy safety is based on the STEP trial series with similar participant numbers. Both trials showed comparable adverse event profiles:
- Most common side effects (affecting over 1 in 10 people): nausea, diarrhoea, vomiting, constipation, and stomach pain
- Very serious side effects are rare in both — though both carry a small risk of pancreatitis and gallbladder complications
- Injection site reactions occur in 5–10% of users with both medications
- Hair loss (alopecia) was reported in around 3% of Monjaro users — less commonly reported with Wegovy
The key difference is mechanism: Monjaro's dual GIP receptor activation may enhance appetite suppression, while Wegovy's single GLP-1 pathway is slightly more established in long-term use. Both require ongoing monitoring by a prescriber, and neither is suitable during pregnancy or breastfeeding.
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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.
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Suitability, Contraindications, and Prescriber Assessment
Safety is not just about side effects — it's about whether a medication suits your individual health profile. Neither Monjaro nor Wegovy is safe for: people under 18, those pregnant or planning pregnancy, people with a personal or family history of medullary thyroid cancer, or those with severe gastroparesis. Both may interact with oral medications including oral contraceptives — any switch requires prescriber guidance.
Your GP or licensed prescriber will assess your medical history, current medications, and weight-related health conditions to determine which (if either) is appropriate for you. Use our free GLP-1 weight loss calculator to understand potential outcomes at different doses, but remember that individual results vary significantly and prescription eligibility is assessed on a case-by-case basis.
- 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 Monjaro safer than Wegovy?
No — clinical trials show comparable safety profiles for both medications. SURMOUNT (Monjaro) and STEP (Wegovy) trials both demonstrated similar rates of common side effects. "Safer" depends on individual health factors, not superiority of one drug over the other. Your prescriber assesses which suits you personally.
What are the most common side effects of Monjaro and Wegovy?
Both cause nausea, diarrhoea, vomiting, constipation, and stomach pain in more than 1 in 10 users. These typically improve within days or weeks as your body adjusts. Hair loss was reported slightly more often with Monjaro (around 3%) than Wegovy. Serious side effects like pancreatitis are rare with both.
Can I switch from Monjaro to Wegovy or vice versa?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart — your prescriber will assess the appropriate transition based on your current dose and response. Never switch without professional guidance.
Are Monjaro and Wegovy safe for everyone?
No. Both are unsuitable for people under 18, those pregnant or breastfeeding, and those with a family history of medullary thyroid cancer. Both may interact with oral medications. Your GP must assess your suitability before prescription — this is why clinical assessment is mandatory.
How do I know if Monjaro or Wegovy is the safer choice for me?
Safety depends on your individual health profile, not the medication itself. Your prescriber will review your medical history, current medications, weight-related conditions, and kidney/liver function to recommend the most appropriate option. Both are prescription-only and require clinical assessment.
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.