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
Free tracker included with every download this week
How Do Weight Loss Jabs Work?
Weight loss jabs—also called slimming injections—work by mimicking natural hormones that regulate appetite and blood sugar. In the UK, the most commonly prescribed options are Wegovy (semaglutide) and Mounjaro (tirzepatide), both available through prescription after a clinical assessment. These medications help reduce food cravings and support feeling fuller after smaller meals, making them useful tools alongside diet and lifestyle changes. Understanding how they work helps you decide if weight management treatment might be right for you.
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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 GLP-1 Weight Loss Injections Work
GLP-1 receptor agonists like Wegovy work by activating natural hormone pathways in your body. GLP-1 (glucagon-like peptide-1) is a gut hormone that signals fullness to your brain after eating. Weight loss injections amplify this signal, which helps reduce appetite and food cravings.
- Slows how quickly food leaves your stomach (gastric emptying), helping you feel fuller for longer
- Signals to your brain that you are satisfied, reducing hunger hormones
- Improves blood sugar regulation, reducing cravings for sugary foods
- Once-weekly injection, prescribed following clinical assessment by a UK healthcare professional
In clinical trials (the STEP programme), Wegovy helped participants lose an average of 15% of their body weight over 68 weeks—though individual results vary significantly. The medication is used alongside reduced-calorie diet and increased physical activity, not instead of them.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Mounjaro's Dual Mechanism — A Different Approach
Mounjaro (tirzepatide) works differently: it's a dual GIP and GLP-1 receptor agonist. Instead of activating just one hormone pathway, it activates two—GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). This dual mechanism may help explain why clinical trial participants lost an average of 15–21% of body weight, slightly more than GLP-1-only medications.
Both Mounjaro and Wegovy are prescription-only in the UK and require assessment by a prescriber to check eligibility. Your BMI, general health, and weight-related conditions determine suitability. Use our free GLP-1 weight loss calculator to understand potential outcomes, and our BMI calculator to check your current health context.
- 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.
Are weight loss jabs safe?
Prescription weight loss injections like Wegovy and Mounjaro are licensed in the UK and have been through clinical trials. Common side effects include nausea and digestive changes, usually mild and temporary. Serious side effects are rare but possible—your GP or prescriber will assess whether the medication is safe for you individually.
How long do weight loss jabs take to work?
Most people notice reduced appetite within 1–2 weeks, but measurable weight loss typically takes 4–8 weeks. Results build over months. Clinical trials lasted 68 weeks (Wegovy) and 72 weeks (Mounjaro). Weight loss is gradual and sustainable when combined with lifestyle changes.
Do you need to keep using weight loss jabs forever?
No. Some people use them short-term (6–12 months) to establish healthy habits, then stop. Others continue longer-term. Your prescriber will review progress and help you decide the best approach for your situation. Weight may return if you stop without maintaining lifestyle changes.
Can I get weight loss jabs on the NHS in the UK?
Yes, prescription weight loss medications are available through NHS specialist weight management services if you meet eligibility criteria (BMI ≥30, or BMI ≥27 with weight-related health conditions). Access varies by region. Waiting lists can be long. Private prescribing is also available through online assessments.
How much weight will I lose on weight loss jabs?
Clinical trials show average weight loss of 15–21% of body weight over several months—but this varies widely. Some people lose more, others less. Weight loss depends on your starting weight, dose, diet, exercise, metabolism, and genetics. Use our GLP-1 weight loss calculator for a personalised estimate based on your situation.
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.