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
What Is a Lose Weight Shot?
A weight loss shot is a prescription-only injection medication that may help support weight loss alongside a reduced-calorie diet and increased physical activity. In the UK, the most commonly prescribed weight loss shots are GLP-1 receptor agonist medications like Wegovy (semaglutide) and Mounjaro (tirzepatide). These injections are not cosmetic procedures — they are licensed pharmaceutical medications that require a clinical assessment from a UK-licensed prescriber before use. Individual results vary significantly, and weight loss is never assured.
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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 Do Weight Loss Injections Work?
Weight loss injections work by mimicking natural hormones that regulate appetite and blood sugar. Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates the GLP-1 hormone pathway to help you feel fuller after smaller meals, reduce food cravings, and slow how quickly food leaves your stomach. Mounjaro (tirzepatide) works differently by activating both GIP and GLP-1 pathways, offering dual hormone action.
In the STEP-1 clinical trial, participants using Wegovy 2.4 mg lost an average of approximately 15% of their body weight over 68 weeks, alongside reduced-calorie diet and increased physical activity. Mounjaro clinical trials (SURMOUNT-1) showed average weight loss of 15–21% depending on dose. However, results vary between individuals — some people lose more, others less. These injections are not standalone treatments; they work best combined with lifestyle changes including sustainable dietary modifications and regular physical activity.
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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.
UK Eligibility and Access
In the UK, weight loss shots may be prescribed to adults with a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. South Asian, Chinese, Black African, and Black Caribbean adults may be eligible at lower BMI thresholds due to higher health risks at these weights.
You can access weight loss injections through NHS specialist weight management services (tier 3/4) following GP referral, though waiting lists vary by region. Private access is available through GPhC-registered online prescribing services following an online clinical assessment — no NHS referral required. All weight loss shots are prescription-only; you cannot buy them without a valid prescription from a UK-licensed prescriber. Our GLP-1 weight loss calculator can help you explore potential results at different doses, and use our BMI calculator to check your current health category.
- 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 shots safe?
Weight loss injections are prescription medications licensed in the UK and have undergone rigorous clinical trials. Common side effects include nausea, diarrhoea, and constipation, which usually improve over time. Serious side effects are rare but require urgent medical attention (severe stomach pain, allergic reactions). Always discuss safety with your prescriber before starting treatment, as some people should not use these medications.
How quickly will I lose weight with a weight loss shot?
Safe, sustainable weight loss is typically 0.5–1 kg per week. Most people begin to see results within 4–8 weeks of starting a weight loss injection, though this varies. Clinical trials showed significant results after 12–16 weeks of treatment. Results depend on your starting weight, adherence to reduced-calorie diet, physical activity level, and individual metabolism.
What's the difference between Wegovy and Mounjaro weight loss shots?
Wegovy (semaglutide) is a single GLP-1 agonist; Mounjaro (tirzepatide) activates two hormone pathways (GIP and GLP-1). Both are once-weekly injections. Mounjaro clinical trials showed slightly higher average weight loss (15–21%) compared to Wegovy (approximately 15%), but individual response varies. Both require prescription and clinical assessment.
Can I get a weight loss shot on the NHS?
Yes, GLP-1 medications are approved by NICE for NHS use through specialist weight management services (tier 3/4). Access requires GP referral and assessment. Availability and waiting times vary significantly by region and ICB. Private prescription is also available without NHS referral through online prescribing services, typically costing £129–£320 per month depending on dose.
Will I regain weight when I stop taking a weight loss shot?
Weight regain can occur after stopping any weight loss treatment if lifestyle changes are not maintained. Clinical evidence suggests sustained weight loss requires ongoing lifestyle changes — reduced-calorie diet and regular physical activity — even after stopping medication. Your prescriber or weight management team can advise on long-term sustainability strategies.
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.