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 Weight Loss Jab? UK Prescription Options Explained
A weight loss jab is a prescription injection that can help manage weight when used alongside diet and exercise. In the UK, three main medications are available: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). These are GLP-1 receptor agonist medications—meaning they work with your body's natural appetite-control hormones. They're only available on prescription following a clinical assessment, and eligibility depends on your BMI and overall health. Many people ask about weight loss injections as part of their weight management journey, and understanding your options is an important first step.
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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.
UK-Licensed Weight Loss Injections and How They Work
Three prescription-only weight loss medications are currently licensed in the UK. Mounjaro (tirzepatide) is a once-weekly injection that activates two hormone pathways—GIP and GLP-1—to reduce appetite and support feeling fuller after meals. Clinical trials showed participants lost an average of 15–21% of their body weight, though individual results vary. Wegovy (semaglutide) is also a once-weekly injection using the GLP-1 pathway, with average weight loss around 15% in trials. Saxenda (liraglutide) is a daily injection and typically produces modest weight loss results.
All three work best when combined with a reduced-calorie diet and increased physical activity. Weight loss typically isn't immediate—most people see changes over weeks and months. A prescriber will assess whether prescription weight loss medication is appropriate for you based on your individual health, medical history, and BMI. You can use our free GLP-1 weight loss calculator to explore projected outcomes at different doses.
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.
NHS Eligibility and How to Access Weight Loss Medication
To access weight loss medication through the NHS, you typically need a BMI of 30 or above, or 27–29.9 if you have weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds due to higher health risks. Your GP can refer you to a specialist tier 3 weight management service, though waiting times vary significantly by region. Alternatively, private prescribing through online services is available without an NHS referral—you'll have a consultation with a UK-licensed prescriber who assesses your eligibility. Check your BMI using our free BMI calculator above to understand where you stand and have a conversation with your GP about which route might suit you best.
- 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.
What's the difference between Mounjaro and Wegovy?
Both are once-weekly injections for weight management, but Mounjaro (tirzepatide) activates two hormone pathways (GIP and GLP-1), while Wegovy (semaglutide) uses the GLP-1 pathway only. In clinical trials, Mounjaro showed slightly higher average weight loss (15–21%) compared to Wegovy (around 15%), though individual results vary significantly. A prescriber can help determine which suits you better.
Are weight loss jabs available on the NHS?
Yes, Mounjaro and Wegovy are available on the NHS through specialist tier 3 weight management services if you meet eligibility criteria (BMI 30+ or 27+ with health conditions). Access varies by region and waiting lists can be lengthy. You can also access these medications privately through online prescribing without NHS referral.
How quickly do weight loss jabs work?
Weight loss isn't immediate. Most people see changes after several weeks as the medication builds in their system. Doses are increased gradually every 4 weeks, and meaningful weight loss typically becomes noticeable after 8–12 weeks. Sustainable loss continues over months alongside lifestyle changes.
What are the main side effects of weight loss jabs?
Common side effects include nausea, diarrhoea, constipation, and stomach discomfort—especially when starting or increasing doses. Most people find side effects improve over time. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder problems. Always report unexpected symptoms to your GP.
Can I get a weight loss jab if my BMI is below 30?
You may qualify if your BMI is 27–29.9 and you have weight-related health conditions like type 2 diabetes, high blood pressure, sleep apnoea, or cardiovascular disease. South Asian and other heritage groups have lower qualifying thresholds. Your GP or a prescriber can assess whether you meet eligibility criteria based on your individual health profile.
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.