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
If you're taking Mounjaro (tirzepatide) in the UK, sun exposure is an important safety consideration. While Mounjaro itself doesn't directly cause photosensitivity, clinical experience and patient reports suggest some people may experience increased sun sensitivity, skin reactions, or worsening of existing skin conditions with prolonged UV exposure. This guide explains the current evidence, NHS recommendations, and practical sun safety tips for people using this prescription-only weight management injection.
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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.
Sun Sensitivity And Mounjaro — What The Evidence Shows
Mounjaro is a dual GIP and GLP-1 receptor agonist injection prescribed for weight management in eligible UK adults. While photosensitivity is not listed as a common side effect in the official product information, some users and clinical reports have noted increased skin sensitivity or reactions following sun exposure while taking tirzepatide.
- Sun sensitivity may manifest as increased redness, burning, or rashes appearing more quickly than usual
- People with existing skin conditions (psoriasis, eczema, rosacea) may experience flare-ups more easily
- Hair loss (alopecia), a known Mounjaro side effect, may be exacerbated by sun damage to the scalp
- The mechanism is not fully understood but may relate to the medication's metabolic effects
If you notice unusual skin reactions after starting Mounjaro, discuss this with your GP or the prescriber who issued your prescription. Individual responses vary significantly, and your healthcare provider can advise whether your symptoms require any changes to your treatment or sun exposure habits.
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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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Practical Sun Safety Whilst Taking Mounjaro
The NHS recommends sun protection for all UK adults, and this is especially important if you're taking Mounjaro and notice increased skin sensitivity:
- Use SPF 30 or higher broad-spectrum sunscreen daily, reapplied every 2 hours and after swimming
- Wear protective clothing: long sleeves, hats, and sunglasses when possible during peak UV hours (11am–3pm)
- Seek shade during the hottest parts of the day
- Keep your scalp protected if you're experiencing hair loss — UV exposure can damage vulnerable hair follicles
- Stay hydrated — Mounjaro can cause nausea and reduced appetite, and dehydration increases skin sensitivity
Track any skin changes in your progress journal and report them to your prescriber at your next review. Early identification of sun-related reactions helps your healthcare team provide appropriate guidance.
- 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 make you more sensitive to the sun?
Photosensitivity is not listed as a common side effect of Mounjaro in the UK prescribing information. However, some users report increased sun sensitivity or faster skin reactions. If you notice unusual redness, burning, or rashes after sun exposure whilst taking tirzepatide, discuss this with your GP or prescriber for personalised advice.
Can Mounjaro cause sun rash or skin reactions?
Whilst not a known direct side effect, increased skin sensitivity has been reported by some Mounjaro users. This may appear as rashes, redness, or irritation developing more quickly than before. Skin conditions like rosacea or eczema may also flare more easily. Always inform your prescriber of any new skin symptoms.
Should I use a higher SPF if I'm taking Mounjaro?
The NHS recommends SPF 30 or higher for all adults. If you notice increased sun sensitivity while taking tirzepatide, using SPF 50+ and reapplying every 2 hours provides extra protection. Combine this with protective clothing and shade-seeking during peak UV hours (11am–3pm).
Can Mounjaro worsen hair loss in the sun?
Hair loss (alopecia) is a known side effect of Mounjaro. UV exposure can damage hair follicles, so protecting your scalp with a hat or sunscreen is especially important whilst taking this medication. Minimise sun exposure to your scalp, and discuss persistent hair loss with your prescriber.
What should I do if I get a skin reaction whilst on Mounjaro?
If you develop an unusual rash, severe redness, or blistering after sun exposure whilst taking tirzepatide, avoid further sun exposure and contact your GP or the prescriber who issued your Mounjaro prescription. Keep a record of when the reaction occurred and any other symptoms you experienced.
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