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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Semaglutide Versus Mounjaro — Key Differences Explained
If you're exploring weight loss options in the UK, you may have come across both semaglutide and Mounjaro. These are two different prescription-only medications that work through similar but distinct mechanisms. Semaglutide (marketed as Wegovy for weight management) is a GLP-1 receptor agonist, while Mounjaro contains tirzepatide, a dual GIP and GLP-1 receptor agonist. Both are licensed in the UK for adults with obesity or overweight with weight-related health conditions. Understanding how they differ — in how they work, their doses, expected results, and side effects — can help you make an informed decision alongside your GP or healthcare provider.
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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.
How Semaglutide and Mounjaro Work Differently
Semaglutide (Wegovy) activates one hormone pathway: GLP-1 (glucagon-like peptide-1). This helps regulate appetite, slows stomach emptying, and signals fullness to the brain. It's a single-mechanism approach that's been used clinically for several years.
Mounjaro (tirzepatide) activates two pathways: both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1. This dual action may provide a more comprehensive effect on appetite regulation and metabolic control. In the SURMOUNT clinical trials, Mounjaro showed slightly higher average weight loss than semaglutide in separate studies, though individual results vary significantly.
Both are once-weekly injections (administered via pre-filled pen), making them more convenient than daily alternatives. The choice between them depends on individual response, tolerability, and your healthcare provider's clinical assessment.
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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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Doses, Side Effects, and Eligibility Differences
Dosing: Semaglutide (Wegovy) starts at 0.25mg and escalates to a maximum of 2.4mg weekly. Mounjaro starts at 2.5mg and escalates to a maximum of 15mg weekly — the dose ranges are quite different. Side effects are similar for both: nausea, diarrhoea, constipation, and stomach discomfort are very common, especially during dose increases. Serious side effects (pancreatitis, gallbladder issues) are rare but require medical attention.
Eligibility is the same for both: BMI ≥30, or BMI 27–29.9 with a weight-related condition (type 2 diabetes, hypertension, sleep apnoea, cardiovascular disease). South Asian, Chinese, Black African, and Caribbean adults may qualify at lower BMI thresholds. Both require prescription from a UK-licensed prescriber and a clinical assessment before starting.
Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses of either medication — this can help visualise the difference between the two. Your GP or prescriber will assess which is more appropriate based on your medical history, other medications, and individual factors. Both are prescription-only and available through NHS specialist weight management services (where availability varies by region) or private online prescribing following a clinical consultation. Neither is suitable during pregnancy or breastfeeding.
- 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 semaglutide the same as Wegovy and Ozempic?
Semaglutide is the active ingredient in both Wegovy and Ozempic. Wegovy is licensed in the UK specifically for weight management (doses up to 2.4mg weekly). Ozempic is licensed for type 2 diabetes (doses up to 2mg) and is NOT approved for weight loss in the UK. Using Ozempic for weight loss would be off-label. Always ensure your prescription specifies Wegovy for weight management.
Which causes more side effects — semaglutide or Mounjaro?
Side effect profiles are very similar between the two. Nausea, diarrhoea, constipation, and stomach pain are common with both, especially during dose increases. Serious side effects (pancreatitis, gallbladder problems) are rare with both. Individual tolerance varies — some people tolerate one better than the other. Your prescriber can advise based on your medical history.
Will I lose more weight on Mounjaro than semaglutide?
Clinical trials suggest Mounjaro may produce slightly higher average weight loss than semaglutide, but individual results vary considerably. Some people lose more on semaglutide, others on Mounjaro. Weight loss depends on dose, adherence, diet, exercise, individual metabolism, and other health factors. Your prescriber will help determine which is more suitable for you.
Can I switch from semaglutide to Mounjaro or vice versa?
Yes, switches between medications are possible and managed by your prescriber. There's no official conversion chart — the approximate equivalence is rough (0.5mg semaglutide ≈ 5mg tirzepatide), but any switch requires clinical oversight. Never switch without consulting your GP or prescriber first.
How do I access semaglutide or Mounjaro in the UK?
Both are available through NHS specialist tier 3 weight management services (via GP referral — availability varies by region) or private online prescribing following a clinical assessment. Private access is faster but costs £129–£320+ monthly depending on dose. NHS access is free but may involve waiting lists. Your GP can advise on both routes.
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