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 and Sun Exposure — What You Need to Know
Mounjaro (tirzepatide) is a prescription-only GIP and GLP-1 receptor agonist injection used for weight management in UK adults. While tirzepatide itself is not known to cause photosensitivity, some people taking Mounjaro report increased skin sensitivity to sunlight. This guide explains what you should know about sun exposure while using this medication and how to protect your skin safely.
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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.
Does Mounjaro Increase Sun Sensitivity?
Mounjaro (tirzepatide) is not documented in clinical trials as a photosensitising medication — meaning it doesn't directly increase your skin's reaction to ultraviolet (UV) light in the way some antibiotics or other drugs do. However, individual experiences vary, and some users report increased sensitivity to sun or feeling more prone to sunburn while taking this prescription weight management injection.
This variation may relate to several factors: gradual weight management can temporarily affect skin health and barrier function; dehydration (a common side effect of Mounjaro) can reduce skin's resilience; or pre-existing skin conditions may become more noticeable during treatment. If you notice unusual skin reactions after starting Mounjaro, discuss this with your GP or prescriber — they can assess whether it's related to the medication or another factor.
- Sun sensitivity is not a listed side effect in Mounjaro clinical data
- Individual responses to sun exposure may vary
- Dehydration can worsen skin's sun tolerance
- Always disclose all medications to your prescriber
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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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Sun Protection While Taking Mounjaro
Whether or not you experience increased sun sensitivity, sun protection is essential for all UK adults, especially during spring and summer months when UV index is highest. Use a broad-spectrum SPF 30 or higher sunscreen daily, reapply every two hours if swimming or sweating, and seek shade during peak UV times (11am–3pm).
Stay hydrated while taking Mounjaro — dehydration is common with this medication and can compromise your skin's natural protective barrier. Drink adequate water throughout the day, particularly in warm weather. If you develop unusual skin reactions, redness, or blistering after sun exposure, contact your GP promptly. Your prescriber needs to know about any new or worsening symptoms to ensure the medication remains suitable for you.
- 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 Mounjaro a photosensitising medication?
Mounjaro (tirzepatide) is not documented as a photosensitising drug in clinical trials, meaning it doesn't directly increase UV light sensitivity. However, individual experiences vary — some users report increased sun sensitivity, which may relate to dehydration, gradual weight management, or other factors rather than the medication itself.
Should I avoid sun exposure while taking Mounjaro?
No, you don't need to avoid sun entirely, but you should use proper sun protection. Wear SPF 30+ sunscreen daily, reapply every two hours, wear protective clothing, and seek shade during peak UV times (11am–3pm). This applies whether or not you take Mounjaro.
Can Mounjaro cause sunburn more easily?
Mounjaro itself doesn't cause sunburn, but dehydration — a common side effect — can weaken your skin's barrier function and make sunburn more likely. Staying well-hydrated while taking this prescription injection helps protect your skin and supports overall health.
What should I do if I develop a skin reaction while on Mounjaro?
Contact your GP if you notice unusual redness, blistering, or severe reactions after sun exposure. Your prescriber needs to know about new symptoms to assess whether they're related to the medication or another cause, and to ensure Mounjaro remains appropriate for you.
Does tirzepatide interact with sunscreen?
No, tirzepatide does not interact with sunscreen. You should use sunscreen normally while taking Mounjaro. In fact, proper sun protection is especially important because dehydration (a side effect of this GLP-1 and GIP agonist) can affect skin health.
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