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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Are weight loss jabs safe?
Weight loss jabs—commonly referred to as GLP-1 injections—are prescription-only medicines that have been clinically tested and approved by UK medicines regulators. The most well-known are Wegovy (semaglutide) and Mounjaro (tirzepatide). Like all medications, they carry potential side effects, but clinical evidence suggests they are safe when prescribed appropriately and used alongside lifestyle changes. This guide explains what the evidence shows, who can use them safely, and what to expect.
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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.
How safe are weight loss injections based on clinical evidence?
Weight loss jabs are prescription-only medicines approved by the UK's Medicines and Healthcare products Regulatory Agency (MHRA), which means they have undergone rigorous safety testing before approval. Wegovy (semaglutide) was evaluated in the STEP clinical trials, which involved thousands of participants over 68 weeks. These trials showed that while side effects were common—particularly nausea, diarrhoea, and constipation—most were mild to moderate and decreased over time as the body adjusted.
Mounjaro (tirzepatide) underwent similar rigorous testing in the SURMOUNT trials. Both medications work by activating natural hormone pathways that regulate appetite and blood sugar. The evidence demonstrates that serious adverse events are rare when these medications are used in eligible patients and prescribed through proper clinical assessment.
- Side effects typically occur in the first few weeks of treatment
- Most side effects resolve as your body adjusts to the medication
- Serious complications are uncommon in eligible patients
- Regular prescriber check-ins help monitor your safety
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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
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Who should and shouldn't use weight loss injections?
Not everyone is a suitable candidate for weight loss jabs. Your GP or licensed prescriber will assess your individual health before prescription. Adults with a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure, may be eligible. However, these injections are not suitable during pregnancy, breastfeeding, or if you have a personal or family history of thyroid cancer or multiple endocrine neoplasia.
If you take insulin or certain diabetes medications, your prescriber may need to adjust your doses. The safety of weight loss jabs depends entirely on proper clinical assessment and ongoing monitoring. Use our free BMI calculator above to check your current BMI, then discuss prescription weight loss options with your GP or a licensed prescriber who can evaluate whether this medication is right for your specific health situation.
- 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 weight loss jabs?
Nausea, diarrhoea, vomiting, and constipation are the most frequent side effects, affecting more than 1 in 10 users in clinical trials. Most are mild to moderate and tend to improve within the first few weeks as your body adjusts. Taking the injection on the same day each week and eating smaller, lower-fat meals can help minimise these effects.
Can weight loss jabs cause serious health problems?
Serious side effects are uncommon in eligible patients. However, seek immediate medical attention if you experience severe stomach pain (possible pancreatitis), gallbladder problems, serious allergic reactions, or changes in vision. This is why clinical assessment before prescription and regular prescriber check-ins are essential for safe use.
Are weight loss jabs the same as Ozempic?
No. Both Wegovy and Ozempic contain semaglutide, but Ozempic is licensed for type 2 diabetes management—not weight loss in the UK. Wegovy is the licensed weight management version at a higher dose (2.4 mg). Using Ozempic for weight loss would be off-label and is not recommended unless specifically prescribed by a clinician.
How do I know if weight loss jabs are safe for me personally?
Your individual safety depends on your health history, other medications, and current medical conditions. Only a licensed prescriber can assess this properly through a clinical consultation. You cannot determine safety based on general information alone—always discuss prescription weight loss options with your GP or a licensed healthcare professional.
Do weight loss jabs work for everyone?
No. Individual results vary significantly. Clinical trials showed average weight loss of 14–21% of body weight, but some people lose more, some less, and some may not lose significant weight. Weight loss jabs work best alongside a reduced-calorie diet and increased physical activity—they support these changes, not replace them.
Ready to take control of your weight loss journey?
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