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 Does Weight Loss Injection Work?
Weight loss injections work by mimicking hormones your body naturally produces to regulate appetite and blood sugar. In the UK, prescription weight management injections like Mounjaro and Wegovy are available for eligible adults to support weight management alongside lifestyle changes. These medications can help reduce food cravings and support feeling fuller after smaller meals. Understanding how they work helps you decide whether prescription weight loss might be right for you, and our GLP-1 weight loss calculator can help you explore potential outcomes at different doses if you're considering treatment.
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Calculate Your BMI Instantly
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 weight loss injections are currently licensed in the UK for weight management in adults:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist, given once weekly. It activates two natural hormone pathways to regulate appetite, slow stomach emptying, and support fat utilisation.
- Wegovy (semaglutide) — a GLP-1 receptor agonist, given once weekly. It signals to your brain that you are full, reduces cravings, and slows how quickly food leaves your stomach.
- Saxenda (liraglutide) — a GLP-1 receptor agonist, given as a daily injection. It works similarly to Wegovy but requires daily dosing rather than weekly.
All three medications mimic GLP-1, a natural hormone produced in your gut after eating. Clinical trials showed average weight loss of 15–21% of body weight with these injections, though individual results vary significantly. They are most effective when combined with a reduced-calorie diet and increased physical activity, not as a substitute for lifestyle changes. Check your BMI using our free BMI calculator to understand your starting point and eligibility.
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 Access vs Private: Finding Out If You're Eligible
Eligibility criteria are the same whether you access treatment through the NHS or privately:
- BMI of 30 or above (obesity), OR
- BMI of 27–29.9 (overweight) with at least one weight-related health condition such as type 2 diabetes, high blood pressure, or heart disease, OR
- Lower BMI thresholds (27.5 and above) for South Asian, Chinese, Black African, or Black Caribbean adults, who face higher health risks at lower weights.
NHS route: Speak to your GP about referral to a specialist tier 3 weight management service. Waiting lists vary by region and can be lengthy.
Private route: Online consultations with UK-licensed prescribers, available without NHS referral. A prescriber will assess your health before issuing a prescription. Use our weight converter to ensure you know your weight accurately in kg, which is used for all clinical assessments.
- 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.
Do weight loss injections work without diet and exercise changes?
No. Weight loss injections work best alongside a reduced-calorie diet and increased physical activity. Clinical trials that showed 15–21% average weight loss all involved participants making lifestyle changes simultaneously. The medication supports these efforts by reducing appetite and cravings, but it is not a substitute for diet and exercise.
How quickly do weight loss injections start working?
Most people notice reduced appetite and food cravings within the first 2–4 weeks. Weight loss typically becomes visible after 4–6 weeks, though this varies between individuals. Doses are gradually increased over several weeks to minimise side effects and allow your body to adjust.
Are weight loss injections safe for long-term use?
Yes, when prescribed and monitored appropriately. Mounjaro and Wegovy have been studied for up to 68 weeks in clinical trials with good safety profiles. However, they do carry side effects (mainly nausea, constipation, and diarrhoea) that should be discussed with your prescriber. Regular check-ins with your healthcare provider are important.
Can I get weight loss injections through the NHS?
Yes, but access is through specialist tier 3 weight management services, not standard GP prescription. You need a GP referral to these services. Availability and waiting times vary significantly by region. Your GP can tell you whether your local NHS service offers these medications and what the current wait is.
What happens when you stop taking weight loss injections?
Weight loss may slow or plateau once you stop taking the medication, and some weight regain is possible if diet and exercise habits do not remain consistent. The medication is intended to support lasting lifestyle changes. Your prescriber can discuss a stopping plan and long-term weight management strategies with you before discontinuing treatment.
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.