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, Wegovy, and Vision Risk: What You Need to Know
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only GLP-1 medications licensed for weight management in the UK. A key safety concern for people considering these injections is the risk of vision changes or blindness. This guide compares what clinical evidence shows about vision risks with each medication, how they differ in mechanism, and what monitoring prescribers recommend. Neither medication directly causes blindness, but vision changes can occur in specific circumstances—and being informed helps you make a confident decision with your healthcare team.
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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.
Clinical Evidence: Vision Safety in SURMOUNT and STEP Trials
Mounjaro (tirzepatide) was tested in the SURMOUNT clinical trials, which included safety monitoring for vision-related adverse events. Vision complications reported were rare and typically related to underlying diabetic retinopathy or rapid blood sugar changes, not the medication itself. Wegovy (semaglutide) underwent similar safety assessment in the STEP trials with comparable findings: vision issues were uncommon and usually associated with pre-existing eye conditions or diabetes-related factors.
- Both medications can cause temporary vision blurriness due to fluid shifts in the eye lens—this usually resolves within weeks
- gradual weight management itself can temporarily affect vision as glucose levels stabilise
- Pre-existing diabetic retinopathy may worsen during initial treatment—this is why screening matters
- Neither medication has a direct mechanism that causes blindness in otherwise healthy eyes
The key difference: Mounjaro's dual GIP+GLP-1 action may affect glucose control slightly differently than Wegovy's single GLP-1 mechanism, but current evidence does not show this translates to different vision risk profiles.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Who Needs Vision Monitoring: Prescriber Assessment and Your Role
Before starting either Mounjaro or Wegovy, your prescriber will assess your eye health history and diabetes status. If you have pre-existing diabetic retinopathy, have poorly controlled blood sugar, or are starting these medications while managing type 2 diabetes, vision monitoring is especially important. You should inform your prescriber of any family history of eye disease, glaucoma, or vision loss.
- Report any new vision blurriness, floaters, flashing lights, or partial vision loss immediately to your GP
- Schedule regular eye tests (as you normally would) whilst taking weight loss medication
- Blood sugar stabilisation during weight loss may temporarily affect vision—this usually improves within 4–6 weeks
Your GP or prescriber will decide whether additional ophthalmology referral is needed based on your individual risk factors. Use our GLP-1 weight loss calculator above to explore whether this medication class is right for you—the results show your personalised eligibility and help you prepare for a prescriber conversation.
- 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 or Wegovy cause blindness?
No. Neither medication directly causes blindness in otherwise healthy eyes. Vision complications are rare and usually occur in people with pre-existing diabetic retinopathy or severe blood sugar dysregulation. Temporary blurriness can happen due to fluid shifts in the eye lens during gradual weight management, but this typically resolves within weeks.
What's the difference in vision risk between Mounjaro and Wegovy?
Current clinical evidence shows no significant difference in vision risk between the two medications. Both are GLP-1 receptor agonists with similar safety profiles. Mounjaro also activates GIP receptors, but this does not translate to higher vision risk in published trials. Individual prescriber assessment determines suitability.
Who should be most cautious about vision changes?
People with pre-existing diabetic retinopathy, uncontrolled type 2 diabetes, or a family history of eye disease should have their eyes monitored more closely. Rapid blood sugar changes during weight loss can temporarily affect vision in these groups. Always inform your prescriber of any eye conditions before starting treatment.
What vision changes should I report immediately?
Contact your GP immediately if you experience sudden blurriness, new floaters, flashing lights in your vision, loss of peripheral vision, or any sudden change in sight. These symptoms warrant urgent assessment, though they are usually not related to the medication itself but to underlying factors like blood sugar shifts.
Do I need an eye test before starting Mounjaro or Wegovy?
Your prescriber will ask about your eye health history during clinical assessment. If you have diabetes, pre-existing eye conditions, or risk factors, your GP may recommend an eye test beforehand. Routine eye tests during treatment are part of normal NHS eye screening—ask your optician about your weight loss medication when you attend.
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