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
Are weight loss injections safe?
Weight loss injections containing GLP-1 medication are prescription-only medicines that have undergone rigorous clinical testing in the UK and internationally. The most common weight loss injection available in the UK is Wegovy (semaglutide), which has been used safely by thousands of people. Like all medications, weight loss injections do carry potential side effects, but serious complications are rare when prescribed appropriately following a clinical assessment. This guide explains what the evidence shows about safety, who should and shouldn't use them, and what to expect.
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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.
What does the clinical evidence show?
Weight loss injections like Wegovy have been extensively studied in large clinical trials. The STEP-1 trial, which followed over 1,600 participants for 68 weeks, demonstrated that Wegovy is effective and has a manageable side effect profile. Common side effects were mostly gastrointestinal — nausea, diarrhoea, vomiting, and constipation — which typically occur during dose escalation and often improve over time as the body adapts.
- Serious adverse events were rare — severe pancreatitis and gallbladder problems occurred in less than 1% of participants
- Most side effects were mild to moderate — affecting digestive comfort rather than overall health
- Discontinuation rates due to side effects were low, suggesting people could tolerate the medication well
- No unexpected safety concerns emerged from extended follow-up data beyond the trial period
The MHRA (UK medicines regulator) approved Wegovy for weight management after thorough safety review. This means the benefits for eligible people outweigh the known risks.
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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.
Who should not take weight loss injections?
Weight loss injections are not suitable for everyone. Certain groups must not use them: people under 18, those who are pregnant or breastfeeding, anyone with a personal or family history of medullary thyroid cancer, and people with severe diabetic complications affecting vision. Those with active gallbladder disease, severe pancreatitis history, or serious gut disorders should avoid these medications. If you take insulin or diabetes medications, dose adjustments may be needed. Your GP or prescriber will assess whether weight loss medication is safe and appropriate for you individually during a clinical assessment.
- 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.
Can weight loss injections cause permanent damage?
No. Weight loss injections like Wegovy are not known to cause permanent organ damage when used as prescribed. Side effects are typically reversible once the medication is stopped. However, very gradual weight management can cause gallstones in some people, which is why steady weight loss (0.5–1 kg per week) is recommended. Always follow your prescriber's guidance.
What's the difference between Wegovy and Ozempic for weight loss?
Both contain semaglutide, but only Wegovy is UK-licensed for weight management. Ozempic is licensed for type 2 diabetes (at lower doses up to 2 mg) and using it off-label for weight loss is not recommended. Wegovy is the regulated, approved choice for weight loss in the UK and reaches the higher dose of 2.4 mg.
Are weight loss injections safer than diet alone?
For most people, lifestyle changes are the safest first approach. However, for those with significant obesity or weight-related health conditions, prescription weight loss injections can be part of a comprehensive plan alongside diet and exercise. Your GP can help decide if medication is appropriate for you by assessing your individual health risks.
Can I use weight loss injections long-term safely?
Yes, weight loss injections can be used long-term under medical supervision. NICE guidance supports ongoing use in UK specialist weight management services. Long-term safety has been demonstrated in extended trial follow-ups. However, you'll need regular GP or prescriber check-ins to monitor your progress and any side effects.
What should I do if I experience serious side effects?
Stop the injection and contact your GP or NHS 111 immediately if you experience severe stomach pain (possible pancreatitis), serious allergic reactions, gallbladder pain, or persistent vomiting. These are rare but require urgent assessment. Minor side effects like nausea usually improve within days or weeks as your body adjusts to the medication.
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.