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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Fat Injections Side Effects — Understanding Your UK Options
Fat injections—also called weight loss injections or slimming injections in the UK—are prescription-only medications that may help with weight management when used alongside diet and exercise. The most commonly discussed options in the UK are Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All work by mimicking natural hormones that regulate appetite and blood sugar. Understanding their side effects, how they differ, and how to access them through NHS or private prescribing is essential for making an informed decision. This guide covers everything you need to know about UK-licensed fat jabs and their safety profile.
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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.
Three UK-Licensed Weight Loss Injections: How They Compare
Three prescription slimming injections are currently licensed for weight management in the UK, each with different mechanisms, dosing schedules, and clinical evidence profiles.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist—the newest option approved for UK weight management. Doses range from 2.5 mg to 15 mg, injected once weekly. Clinical trials (SURMOUNT-1) showed average weight loss of approximately 15–21% of body weight at 72 weeks, though individual results vary significantly. Common side effects include nausea, diarrhoea, constipation, and stomach pain, typically mild to moderate and often improving over time.
Wegovy (semaglutide) is a GLP-1-only receptor agonist, also injected once weekly with doses ranging from 0.25 mg to 2.4 mg. STEP-1 clinical trial data showed average weight loss of around 15% of body weight. Side effects are similar to Mounjaro: nausea, diarrhoea, constipation, and indigestion are most common.
Saxenda (liraglutide) is also a GLP-1 agonist but administered as a daily injection (0.6 mg to 3 mg). Weight loss outcomes are typically more modest than Mounjaro or Wegovy, with average losses around 5–8% of body weight in clinical trials. However, some patients prefer daily dosing as it may help them feel more engaged with treatment.
All three are prescription-only and require clinical assessment before prescribing. None are cosmetic fat-dissolving injections—they are medications that work systemically to regulate appetite and metabolism.
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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.
Accessing Fat Injections in the UK: NHS vs Private Routes
NHS Access: Weight loss injections are available through NHS tier 3 specialist weight management services, but access varies significantly by region and availability. You typically need a referral from your GP, and waiting lists can be lengthy in many areas. NICE has approved tirzepatide (Mounjaro) for use within NHS services, and semaglutide (Wegovy) eligibility is also available through some NHS trusts.
Private Access: Private prescription is available through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber. This route does not require NHS referral and is often faster, though costs range from approximately £129–£320 per month depending on dose and provider.
Eligibility for Both Routes: You must have a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, cardiovascular disease, or sleep apnoea). For South Asian, Chinese, Black African, or Black Caribbean adults, lower BMI thresholds apply. A prescriber will assess whether fat injections are suitable based on your individual health, medications, and medical history. Use our free BMI calculator above to check your starting point, and our GLP-1 weight loss calculator can help estimate potential outcomes at different doses.
- 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 are the most common side effects of fat injections in the UK?
Nausea, diarrhoea, constipation, and stomach pain are very common, affecting more than 1 in 10 people. These side effects typically appear in the first few weeks and often improve as your body adjusts. Less common but possible side effects include headache, fatigue, dizziness, and injection site reactions. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder problems—seek immediate medical attention if these occur.
Which fat injection causes the least side effects—Mounjaro, Wegovy, or Saxenda?
All three have similar side effect profiles because they work on appetite-regulating hormones. Individual tolerance varies greatly—some people have minimal side effects, others experience nausea or constipation. There is no clear evidence that one causes significantly fewer side effects than the others. Your prescriber can help determine which is most suitable based on your medical history and tolerability. Starting at a low dose and gradually increasing typically reduces side effects.
Can I get fat injections on the NHS in the UK?
Yes, weight loss injections are available through NHS specialist tier 3 weight management services. However, availability varies by region and waiting times can be long. You need a GP referral to access NHS services. Private access through GPhC-registered online prescribers is faster but involves private costs. Check with your local NHS integrated care board for eligibility and waiting times in your area.
How long do I need to take fat injections for?
Weight loss injections are typically used for a minimum of 12 weeks to assess effectiveness, but many people continue for 6–12 months or longer. Once you stop taking the medication, weight regain is possible if lifestyle changes (diet and exercise) are not maintained. Your prescriber will review your progress regularly and discuss ongoing treatment goals with you. Track your weight loss journey using our free progress tracker.
Are fat injections safe for people with type 2 diabetes?
Yes, GLP-1 and GIP/GLP-1 medications are safe for people with type 2 diabetes and may actually help improve blood sugar control. However, if you're taking diabetes medications (insulin, sulphonylureas), your prescriber may need to adjust doses because fat injections can lower blood sugar levels. Always inform your prescriber of all current medications before starting treatment. Discuss your diabetes management plan with your GP or diabetes specialist before beginning.
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.