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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Weight Loss Injectable: How It Works in the UK
A weight loss injectable is a prescription-only medication delivered by injection that helps support weight management when used alongside diet and physical activity. In the UK, these medications work by mimicking natural hormones that regulate appetite and blood sugar. The most commonly prescribed weight loss injectables are GLP-1 receptor agonists like Wegovy (semaglutide), which clinical trials have shown may help adults lose an average of 15% of their body weight. Access to prescription weight loss medication through NHS weight management services or private prescribing requires a clinical assessment to determine if it's suitable for you.
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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.
How Weight Loss Injectables Work
Weight loss injectables are GLP-1 receptor agonist medications that activate a natural hormone pathway in your body. GLP-1 (glucagon-like peptide-1) is produced in the gut and signals to your brain that you're full, helping to reduce appetite and food cravings. These medications also slow how quickly food leaves your stomach, which extends the sensation of fullness after meals.
In clinical trials, Wegovy (semaglutide) — the most widely prescribed weight loss injectable in the UK — helped participants achieve meaningful results. The STEP-1 trial showed an average weight loss of approximately 14.9% of body weight over 68 weeks at the maximum dose of 2.4mg, alongside a reduced-calorie diet and increased physical activity.
Use our free GLP-1 weight loss calculator to estimate potential weight loss at different doses. You can also use our BMI calculator to check your current health metrics and understand where you fall within NHS weight categories.
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.
UK Eligibility and Access Routes
To access a weight loss injectable in the UK, you typically need a BMI of 30 or above, or a BMI of 27–29.9 with at least one weight-related health condition (such as type 2 diabetes, high blood pressure, or sleep apnoea). For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds apply — eligibility may start at BMI 27.5 or above due to higher health risks at lower weights.
NHS access: Referral to a specialist tier 3 weight management service; availability varies significantly by region. Private access: Online consultation with a UK-licensed prescriber followed by dispensing through a registered pharmacy. Both routes require clinical assessment and a valid prescription.
- 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.
Is a weight loss injectable the same as insulin?
No. Weight loss injectables like Wegovy are GLP-1 receptor agonists — completely different from insulin. They don't work on blood sugar in the same way. GLP-1 medications support appetite regulation and are prescribed for weight management, whereas insulin manages blood glucose in diabetes. Always ask your prescriber to clarify if you have concerns.
What are the common side effects of weight loss injectables?
Very common side effects include nausea, diarrhoea, constipation, and stomach discomfort — particularly when starting or increasing doses. Most side effects settle within a few weeks as your body adjusts. Serious side effects are rare but include severe stomach pain (possible pancreatitis), gallbladder problems, or allergic reactions. Always report persistent or severe symptoms to your prescriber.
How long do you need to take a weight loss injectable?
The duration depends on your health goals and how well you respond. Some people take weight loss injectables for several months; others continue longer-term. Your prescriber will discuss the right timeframe for you. Stopping the medication may result in weight regain, so lifestyle changes are essential alongside any injectable treatment.
Can I get a weight loss injectable on the NHS?
Yes, through specialist tier 3 weight management services. Your GP can refer you if you meet eligibility criteria (BMI 30+ or BMI 27–29.9 with comorbidities). However, NHS availability varies by region, and waiting lists can be lengthy. Private options are also available through online prescribing if you meet clinical criteria.
What's the difference between Wegovy and Ozempic — aren't they the same?
Both contain semaglutide, but Ozempic is licensed for type 2 diabetes management (up to 2mg) and is NOT licensed for weight loss in the UK. Wegovy is specifically licensed for weight management at higher doses (up to 2.4mg). Using Ozempic for weight loss is off-label. Always request Wegovy if weight management is your goal, and confirm with your prescriber what you're being prescribed.
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.