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
Slimming Injection Side Effects — What You Need to Know
Slimming injections—also called weight loss injections or "fat jabs"—are prescription-only medicines that help regulate appetite and support weight management. In the UK, three GLP-1 weight loss injections are currently licensed: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). These are not cosmetic injections—they're legitimate prescription medications available through NHS specialist services or private prescribing following a clinical assessment. Understanding how they work and what side effects to expect is essential before deciding if slimming injections are right for you.
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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 slimming injections are currently available in the UK, each working as a GLP-1 receptor agonist—a medication class that mimics natural gut hormones:
- Mounjaro (tirzepatide): A dual GIP and GLP-1 agonist, dosed from 2.5 mg to 15 mg once weekly. Clinical trials showed participants lost an average of 15–21% of body weight.
- Wegovy (semaglutide): A GLP-1 agonist, dosed from 0.25 mg to 2.4 mg once weekly. STEP trial data showed average weight loss of around 15% of body weight at maximum dose.
- Saxenda (liraglutide): A GLP-1 agonist, dosed from 0.6 mg to 3 mg once daily (the only daily injection option). Approved for weight management in UK adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions.
All three are prescription-only medicines requiring a clinical assessment before dispensing. Individual results vary significantly based on lifestyle changes, genetics, and adherence to treatment.
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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.
How Slimming Injections Work: The GLP-1 Mechanism
GLP-1 (glucagon-like peptide-1) is a natural hormone your gut produces after eating. Weight loss injections activate GLP-1 receptors in your brain and digestive system, helping you feel fuller sooner and reducing hunger signals. This mechanism supports a reduced calorie intake without requiring willpower alone.
These medications slow how quickly food leaves your stomach, helping you feel satisfied after smaller meals. They work best alongside a reduced-calorie diet and increased physical activity—they're not a replacement for lifestyle changes. Try our free GLP-1 weight loss calculator above to see projected weight loss at different doses, and use our BMI calculator to check your current eligibility for prescription weight loss treatment.
- 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 slimming injections?
The most common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain. These are usually mild and tend to improve as your body adjusts to the medication. Starting at a low dose and increasing gradually helps minimise nausea. If side effects persist or worsen, speak to your prescriber—they may adjust your dose.
Are slimming injections safe to use long-term?
Mounjaro, Wegovy, and Saxenda have been studied in clinical trials lasting up to 68–72 weeks with safety data available. Long-term safety data beyond this period is still emerging. Your prescriber will monitor your health regularly and assess whether continued use is appropriate for you. Always report any new symptoms to your healthcare provider.
Who is eligible for prescription weight loss injections in the UK?
Adults with a BMI of 30 kg/m² or above are generally eligible. Those with a BMI of 27–29.9 kg/m² may also qualify if they have 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 have lower eligibility thresholds due to higher health risks at lower BMI. A prescriber must assess you individually.
Can you stop using slimming injections whenever you want?
Yes, you can stop at any time—these are not medications you become dependent on. However, weight often returns gradually after stopping unless lifestyle changes are maintained. Your prescriber can advise on the safest way to stop, which may involve gradually reducing the dose rather than stopping suddenly.
What's the difference between slimming injections and cosmetic fat-dissolving injections?
Weight loss injections like Mounjaro and Wegovy are prescription medicines that work systemically on appetite regulation. Cosmetic fat-dissolving injections (such as aqualyx) are local treatments injected directly into fat deposits and work differently. Weight loss injections are medical treatments requiring clinical assessment; cosmetic injections are aesthetic procedures. Never confuse the two.
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.