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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Diet Injections Side Effects — UK Weight Loss Options
Weight loss injections, commonly called slimming injections or fat jabs in the UK, are prescription-only medicines that can help support weight management. Three main options are licensed in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All work by mimicking natural hormones that regulate appetite, but they differ in mechanism, dosing frequency, and side effects. This guide explains what to expect from each option and how to access them through NHS or private prescribing.
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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.
Mounjaro vs Wegovy vs Saxenda — Mechanisms and Side Effects
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two hormone pathways simultaneously. It's administered once weekly at doses from 2.5mg to 15mg. Common side effects include nausea, diarrhoea, constipation, and stomach pain, typically most pronounced in the first few weeks. Clinical trials showed average weight loss of around 15–21% of body weight at maximum dose.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, with a single mechanism of action. It's also once-weekly, escalating from 0.25mg to 2.4mg. Side effects are similar to Mounjaro: nausea, vomiting, diarrhoea, constipation, and indigestion. Average weight loss in clinical trials was approximately 15% of body weight.
Saxenda (liraglutide) is a daily injection (not weekly), dosed from 0.6mg to 3mg. It's also a GLP-1 agonist. Side effect profile is comparable, though daily dosing may suit some people better than weekly injections. Average weight loss data shows similar results to Wegovy when dosed appropriately.
All three are prescription-only medicines requiring clinical assessment. Individual results vary significantly, and side effects typically improve within 4–8 weeks as your body adjusts.
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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.
NHS Access vs Private Prescribing in the UK
NHS access to prescription weight loss injections is available through specialist tier 3 weight management services. Your GP can refer you, but eligibility and waiting times vary dramatically by region. Some NHS services have waiting lists of 12–24 months. You must have a BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions like type 2 diabetes or high blood pressure.
Private prescribing is available through GPhC-registered online prescribing services following a clinical assessment. This route is faster (often 1–2 weeks) and doesn't require NHS referral, but costs vary from £129–£320 per month depending on dose and provider. Both routes require a valid prescription from a UK-licensed prescriber. Use our free BMI calculator to check if you may be eligible, and consult your GP or a licensed prescriber to discuss which option suits you.
- 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 diet injections in the UK?
Nausea, diarrhoea, constipation, and stomach pain are the most frequently reported side effects across all UK-licensed weight loss injections. These typically appear in the first 1–2 weeks and often improve significantly by week 4–8 as your body adjusts. Taking injections with food, staying hydrated, and avoiding fatty or spicy meals can help minimise nausea.
Which diet injection causes fewer side effects — Mounjaro or Wegovy?
Both have similar side effect profiles because they target the same hormone systems. Mounjaro's dual mechanism (GIP + GLP-1) may be slightly more effective for weight loss in some people, but clinical data doesn't show it has fewer side effects than Wegovy. Individual tolerance varies — what suits one person may not suit another. Your prescriber can help choose based on your health history.
Can I get slimming injections on the NHS in the UK?
Yes, but availability is limited. NICE has approved tirzepatide and semaglutide for NHS use through specialist weight management services (tier 3). Access depends on your regional ICB, eligibility criteria (BMI 30+ or 27–29.9 with comorbidities), and current waiting lists. Private prescribing is faster and doesn't require NHS referral, though it involves a cost.
How long do side effects last with prescription weight loss injections?
Most side effects are worst in the first 1–2 weeks and typically improve significantly by week 4–8. Some people experience mild nausea or digestive changes throughout treatment, but severe side effects are uncommon after the initial adjustment period. If side effects persist or worsen, contact your prescriber — dose adjustment or switching medications may help.
Are all weight loss injections prescription-only in the UK?
Yes. Mounjaro, Wegovy, and Saxenda are all prescription-only medicines in the UK. They cannot be purchased over the counter or without a clinical assessment from a UK-licensed prescriber. If you see these medications offered without a prescription, they are not legitimate UK supplies. Always use GPhC-registered pharmacies or NHS services.
Ready to take control of your weight loss journey?
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