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 vs Wegovy — Vitamins and Nutrient Considerations
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work differently and may affect nutrient absorption in different ways. Understanding the distinction between these dual GIP/GLP-1 and single GLP-1 mechanisms, along with vitamin and mineral considerations, can help you and your prescriber decide which might be most suitable for your health. This guide explains the key differences and what you need to know about vitamin supplementation while using either medication.
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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.
How Mounjaro and Wegovy Differ — Mechanism and Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in the body. This dual mechanism may offer stronger appetite suppression and metabolic effects. In the SURMOUNT-1 clinical trial, participants on tirzepatide achieved average weight loss of approximately 15–21% of body weight over 72 weeks, with the 15 mg dose showing the highest results.
Wegovy (semaglutide) is a GLP-1-only receptor agonist — it activates a single hormone pathway. The STEP-1 clinical trial showed average weight loss of approximately 14.9% of body weight at 68 weeks on the 2.4 mg dose. Both medications slow gastric emptying and reduce appetite, but Mounjaro's dual action may produce slightly stronger effects for some individuals. However, individual results vary significantly, and what works best depends on your personal health profile, not trial averages. Both require a UK prescription and clinical assessment before being prescribed.
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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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Vitamin and Nutrient Absorption — What You Should Know
Both Mounjaro and Wegovy slow how quickly food moves through your stomach (gastric emptying), which can affect how your body absorbs certain vitamins and minerals. This effect typically reduces over time as your body adapts. Your GP or prescriber may recommend vitamin supplementation, particularly vitamin B12, iron, and calcium, depending on your baseline levels and diet.
Neither medication directly depletes vitamins — rather, reduced food intake and slower digestion may limit absorption. A balanced diet including protein-rich foods, leafy greens, and fortified foods helps maintain nutrient status. Blood tests can check your B12 and iron levels before starting either medication and during treatment. Always discuss supplementation with your prescriber rather than self-supplementing, as individual needs vary. Use our free BMI calculator to check your starting point, and our GLP-1 weight loss calculator can help you estimate potential outcomes with either medication.
- 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 more effective than Wegovy for weight loss?
Clinical trials show Mounjaro achieved slightly higher average weight loss (15–21%) compared to Wegovy (14.9%), but individual results vary significantly. Both are effective; the best choice depends on your health profile, side effect tolerance, and prescriber assessment. Neither assures specific outcomes.
Do I need vitamin supplements while taking Mounjaro or Wegovy?
Both medications may reduce nutrient absorption through slower gastric emptying. Your GP may recommend B12, iron, or calcium supplementation. Blood tests before and during treatment help guide decisions. Never self-supplement without professional advice.
Which medication is available on the NHS in the UK?
NICE has approved tirzepatide (Mounjaro) and semaglutide (Wegovy) for specialist NHS weight management services. Availability varies by region and waiting lists can be lengthy. Both are also available privately through UK-licensed prescribers following online clinical assessment.
What's the key difference between Mounjaro and Wegovy mechanisms?
Mounjaro is a dual GIP and GLP-1 agonist (two hormone pathways), while Wegovy is a GLP-1-only agonist (one pathway). The dual mechanism may produce stronger appetite suppression, but both require prescription and clinical suitability assessment.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching between medications must be managed by your prescriber. Approximate dose equivalences exist (Wegovy 2.4 mg ≈ Mounjaro 10–15 mg), but any switch requires individualised medical assessment and a new prescription. Never switch without professional guidance.
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