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
Side Effects Of Slimming Injections
Slimming injections, also called weight loss injections or "fat jabs", are prescription medicines used to help adults manage excess weight in the UK. These are not cosmetic injections — they're GLP-1 receptor agonist medications that work by regulating appetite and helping you feel fuller after eating. Before starting any prescription slimming injection, it's important to understand the potential side effects. This guide explains what you might experience, which effects are common, and when to seek medical advice.
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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.
Three UK-Licensed Slimming Injections
Three prescription slimming injections are currently licensed for weight management in the UK. Mounjaro (tirzepatide) is a once-weekly injection that works on two hormone pathways — it's the newest option and clinical trials showed average weight loss of around 15–21% of body weight. Wegovy (semaglutide) is also a once-weekly injection that activates GLP-1 receptors, with clinical trials showing approximately 15% average weight loss. Saxenda (liraglutide) is a daily injection with similar GLP-1 mechanisms. All three are prescription-only medicines requiring clinical assessment before use. Each has its own side effect profile, though nausea, diarrhoea, and constipation are common across all three. Your prescriber will discuss which NHS weight loss injections or private options suit your health situation.
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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 And Why Side Effects Occur
Weight loss injections work by mimicking natural hormones that control appetite and digestion. GLP-1 weight loss medications slow how quickly food leaves your stomach and signal to your brain that you're full — this is why nausea and changes to appetite are frequent side effects. Your digestive system is adjusting to a new chemical signal. Most side effects are mild and temporary, settling within days or weeks as your body adapts. Serious side effects like severe stomach pain (possible pancreatitis) or gallbladder problems are rare but require urgent medical attention. Use our free BMI calculator to establish your baseline health metrics before starting treatment, and track any changes during your weight loss journey with our progress tracker tool.
- 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?
Nausea, diarrhoea, vomiting, constipation, and stomach pain are very common — affecting more than 1 in 10 people. Indigestion, fatigue, dizziness, and reduced appetite are also common. Most settle within the first few weeks as your body adjusts to the medication. Starting on a low dose helps minimise early side effects.
Are slimming injections safe in the UK?
Yes, when prescribed appropriately through NHS specialist services or following a clinical assessment by a licensed prescriber. All three UK-licensed options have been clinically tested and approved by the MHRA. Side effects are mostly mild and manageable. Always inform your prescriber of all existing health conditions and medications.
When should I stop taking slimming injections and see my GP?
Seek urgent medical advice if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions, gallbladder pain, or changes in vision. Contact your prescriber about any side effect that worsens or doesn't improve after 2–3 weeks.
Do side effects go away over time?
Yes, in most cases. Gastrointestinal side effects typically improve within 1–4 weeks as your body adjusts. Some people experience hair loss — this is usually temporary and related to gradual weight management rather than the medication itself. Staying hydrated and eating small, frequent meals can help minimise nausea.
Can I switch between different slimming injections if side effects are bad?
Your prescriber may discuss switching options if side effects are intolerable, though this isn't always straightforward. Different injections may have slightly different tolerability profiles. Never stop or switch medications without consulting your prescriber — they'll help find the best option for your individual health needs.
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.