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 Blindness Risk vs Wegovy — What the Evidence Shows
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) carry a potential risk of diabetic retinopathy complications in people with existing diabetes, though blindness itself is extremely rare with either medication. This guide compares the eye safety profiles of both GLP-1 and dual GIP/GLP-1 receptor agonists based on UK clinical evidence, mechanism differences, and what you should monitor. Both medications require a clinical assessment before prescription — your GP or prescriber will evaluate your individual risk factors, including any existing diabetes or eye conditions. Use our free BMI calculator to assess your eligibility for either medication.
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Calculate Your BMI Instantly
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 Evidence: SURMOUNT vs STEP Trial Data on Eye Safety
Mounjaro (tirzepatide) was studied in the SURMOUNT-1 clinical trial across approximately 2,500 adults over 72 weeks. The trial monitored for adverse events including diabetic retinopathy, though blindness was not a reported outcome. Among participants, there were rare cases of diabetic retinopathy complications in people with pre-existing type 2 diabetes.
Wegovy (semaglutide) was evaluated in the STEP-1 trial with similar safety monitoring. Like Mounjaro, blindness is not a recognised clinical outcome with Wegovy. However, people with diabetes can experience worsening retinopathy when blood sugar drops rapidly — a theoretical concern with both medications.
- Neither drug directly causes blindness in clinical trials
- Both carry a theoretical risk of diabetic retinopathy complications in pre-existing diabetes
- Rapid blood sugar improvement may unmask or worsen existing diabetic eye disease
- Eye safety depends on individual baseline health status and diabetes control
The key difference is mechanism: Mounjaro is a dual GIP/GLP-1 agonist, while Wegovy is a GLP-1-only agonist. This dual mechanism may affect blood sugar control differently, though both support healthier glucose levels.
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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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Who Should Be Monitored and Individual Suitability Assessment
If you have pre-existing diabetes or diabetic retinopathy, you must inform your prescriber before starting either Mounjaro or Wegovy. UK NICE guidance recommends baseline eye assessments for anyone with diabetes starting weight loss medications, as rapid metabolic improvements can occasionally trigger temporary vision changes.
Your prescriber will assess:
- Whether you have type 1 or type 2 diabetes
- Current blood sugar control and HbA1c levels
- Any history of diabetic retinopathy or eye conditions
- Current diabetes medications (insulin or sulphonylureas may need adjustment)
- Overall cardiovascular and metabolic health
Neither medication is contraindicated in diabetes, but individualised clinical assessment is essential. Both require a prescription and clinical decision — neither is suitable for self-diagnosis or self-prescription. Your GP can refer you to specialist weight management services where these decisions are made with your complete health picture.
- 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.
Does Mounjaro cause blindness?
No. Mounjaro does not directly cause blindness. However, in people with pre-existing diabetic retinopathy, rapid blood sugar improvement may temporarily worsen vision — a known complication of any diabetes treatment. This is not blindness but a potentially reversible change requiring eye monitoring.
Can Wegovy cause eye problems or blindness?
Wegovy (semaglutide) does not cause blindness in clinical trials. Like Mounjaro, it carries a theoretical risk of diabetic retinopathy complications in people with existing diabetes because rapid metabolic improvement can unmask pre-existing eye disease. Individual risk depends on baseline diabetes control.
Which is safer for your eyes — Mounjaro or Wegovy?
Clinical evidence shows no significant difference in eye safety between the two. Both are safe for most people. Risk is specific to those with pre-existing diabetes or retinopathy. Your prescriber will assess your individual safety profile — this is why clinical assessment before prescription is mandatory for both.
Should I get an eye test before starting Mounjaro or Wegovy?
If you have diabetes or a family history of diabetic retinopathy, yes. UK specialist weight management services typically arrange baseline eye assessments. Your GP can refer you to these services, where full safety checks — including eye health — are completed before prescription.
What vision changes might happen on Mounjaro or Wegovy?
Temporary blurred vision or floaters occasionally occur, typically during rapid blood sugar improvement in people with diabetes. These are usually reversible. Persistent vision changes must be reported to your GP immediately. Neither medication causes permanent blindness in clinical practice, but eye monitoring is important for people with diabetes.
Ready to take control of your weight loss journey?
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