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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How Does Wegovy Work For Weight Loss?
Wegovy (semaglutide) is a prescription-only weekly weight loss injection that works by activating natural hormone pathways in your body to reduce appetite and support feeling fuller after meals. As a GLP-1 receptor agonist, it mimics a hormone your gut produces naturally in response to food. In clinical trials, Wegovy helped participants lose an average of around 15% of their body weight alongside a reduced-calorie diet and increased physical activity. Individual results vary significantly, and it's important to understand both how it works and how it differs from other weight loss medications available in the UK.
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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.
The Science: GLP-1 Mechanism vs Dual GIP+GLP-1
Wegovy works as a GLP-1 receptor agonist — it activates only the GLP-1 hormone pathway. GLP-1 (glucagon-like peptide-1) is a natural hormone your gut releases after eating. It signals your brain that you're full, slows how quickly food leaves your stomach, and helps regulate blood sugar. By mimicking and amplifying this signal, Wegovy may help reduce hunger and food cravings.
This differs from Mounjaro (tirzepatide), which is a dual agonist — it activates both GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) pathways simultaneously. In clinical trials, Mounjaro users experienced slightly higher average weight loss (up to 21% body weight) compared to Wegovy users (around 15%), though individual results vary considerably.
- Wegovy: Single GLP-1 pathway activation
- Mounjaro: Dual GLP-1 + GIP pathway activation
- Both are once-weekly injections requiring prescription
- Both require clinical assessment before being prescribed
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Clinical Evidence and Individual Suitability
The STEP-1 clinical trial showed Wegovy achieved an average weight loss of approximately 14.9% body weight over 68 weeks, studied alongside a reduced-calorie diet and increased physical activity. However, results varied — some participants lost more, others less. This variation is normal and reflects differences in metabolism, adherence, lifestyle changes, and individual biology.
Whether Wegovy or Mounjaro is more suitable depends on your individual circumstances. Both are eligible for UK adults with a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure. Your GP or a licensed prescriber will assess which medication (if any) is appropriate following a clinical assessment. Use our free GLP-1 weight loss calculator to understand potential outcomes, and try our BMI calculator to check your current status.
- 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.
How quickly does Wegovy start working?
Wegovy typically begins affecting appetite within the first week, though the most significant weight loss usually occurs over weeks 4–8 and beyond. You'll start on a low dose (0.25 mg) and gradually increase weekly. Full effects develop over several months as your body adjusts and you reach higher doses.
What are the main side effects of Wegovy in the UK?
Very common side effects include nausea, diarrhoea, vomiting, and constipation, particularly when starting or increasing doses. Most side effects improve within days to weeks as your body adapts. Less common but serious side effects include severe stomach pain (possible pancreatitis) — seek immediate medical attention if this occurs.
Is Wegovy available on the NHS in the UK?
Wegovy is available through NHS specialist weight management services (tier 3), but access varies significantly by region and waiting lists can be long. It's also available privately through GPhC-registered online pharmacies following a clinical assessment. Your GP can refer you to NHS services or discuss private options.
Why does Mounjaro cause more weight loss than Wegovy if both are GLP-1 medications?
Mounjaro activates two hormone pathways (GLP-1 plus GIP), while Wegovy activates only GLP-1. The dual mechanism in Mounjaro may provide additional appetite suppression and metabolic support. However, this doesn't mean Mounjaro is "better" — individual response varies, and Wegovy remains effective for many people.
Can you switch from Wegovy to Mounjaro or vice versa?
Switching between medications is possible but must be managed by your prescriber. There's no official conversion chart — your prescriber will assess whether a switch is appropriate based on your response, side effects, and individual circumstances. Never change medications without professional guidance.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.