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 Sight Loss vs Wegovy — What UK Adults Need to Know
Both Mounjaro and Wegovy are prescription weight loss injections licensed in the UK, and both have been associated with vision changes in some users. This guide compares the clinical evidence, mechanisms, and what the difference means for your health. If you're considering a weekly weight loss injection, understanding how these medications work and their potential side effects is essential for making an informed decision with your prescriber. Use our free BMI calculator to check your eligibility, then discuss the right option for you with a licensed healthcare professional.
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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 Evidence and Trial Data Comparison
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways to regulate appetite and blood sugar. The SURMOUNT-1 trial, which tested Mounjaro doses up to 15 mg weekly, reported vision changes as a side effect in some participants. Wegovy (semaglutide) is a single GLP-1 receptor agonist — it activates one pathway — with a maximum dose of 2.4 mg weekly. The STEP trials for Wegovy similarly documented vision-related side effects, though frequency varies.
Vision changes linked to both medications are typically temporary and often related to blood sugar fluctuations or mild dehydration rather than permanent eye damage. However, both injections can affect the eyes, particularly in people with existing diabetic retinopathy (eye damage from diabetes). The trial data shows vision changes are uncommon but worth monitoring, especially if you have diabetes or pre-existing eye conditions. Your prescriber will assess your individual risk during your clinical assessment for a prescription weight loss injection.
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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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Mechanism Differences and Individual Suitability
The key difference is mechanism: Mounjaro's dual action on GIP and GLP-1 pathways may have different effects on metabolism and eye health compared to Wegovy's single GLP-1 action. Neither mechanism has proven superiority for vision safety — individual response varies significantly. Suitability depends on:
- Your medical history, including any diabetes or eye conditions
- Other medications you take
- Your individual weight loss goals and health needs
- Prescriber assessment of risk versus benefit
Neither medication is "safer" for vision — both require careful monitoring. If you have concerns about vision changes, discuss them openly with your GP or the prescriber assessing you. Use our free weight loss calculator to explore potential outcomes, then book a clinical assessment to discuss which prescription medication, if any, is appropriate for your circumstances.
- 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.
Can Mounjaro cause permanent sight loss?
Sight loss directly caused by Mounjaro is extremely rare. Vision changes reported in trials are usually temporary and linked to blood sugar shifts or dehydration. However, if you have pre-existing diabetic retinopathy, both Mounjaro and Wegovy can worsen it. Always inform your prescriber of any eye conditions before starting treatment.
Is Wegovy safer for the eyes than Mounjaro?
Clinical trial data does not show Wegovy is safer for vision than Mounjaro. Both medications can affect the eyes in some users. Vision safety depends on your individual health, medical history, and how your body responds — not on one being inherently safer. Your prescriber will assess this during your clinical assessment.
What vision changes do people report with these injections?
Reported changes include temporary blurred vision, changes in prescription strength, and rarely, more serious complications. Most are mild and resolve when blood sugar stabilises. If you experience sudden vision changes, floaters, or eye pain while taking either medication, contact your GP or eye care provider immediately.
Who should not take Mounjaro or Wegovy due to eye concerns?
People with active or severe diabetic retinopathy, recent eye surgery, or certain retinal conditions may be advised against these medications. Your GP will review your eye health history and may refer you to an eye specialist before prescribing. A thorough clinical assessment protects your vision safety.
How do I monitor my vision while on these weight loss injections?
Have regular eye checks with your optician, report any vision changes immediately to your prescriber, stay well hydrated, monitor blood sugar if diabetic, and attend all follow-up appointments. Keep an open dialogue with both your GP and eye care provider about any concerns during your weight loss treatment.
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.