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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Which weight loss injection is safest for you?
Three prescription weight management injections are currently licensed in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Each has been approved by the MHRA and tested in clinical trials, but safety and suitability vary between individuals. The "safest" option depends on your personal health profile, medical history, and how your body responds — something only a prescriber can assess through a clinical evaluation. This guide explains the UK-licensed options and how to find out which might be appropriate 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 their safety profiles
All three prescription weight loss injections available in the UK have undergone rigorous safety testing and clinical trials before approval:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist, approved for weight management. SURMOUNT clinical trials showed average weight loss of 15–21% of body weight. Side effects are typically mild-to-moderate gastrointestinal symptoms (nausea, diarrhoea, constipation), which often improve over time.
- Wegovy (semaglutide) — a GLP-1 receptor agonist, approved for weight management. STEP trials showed average weight loss of approximately 15% of body weight. Similar side effect profile to Mounjaro, with nausea and digestive changes most common.
- Saxenda (liraglutide) — a GLP-1 receptor agonist given as a once-daily injection. Also MHRA-approved for weight management with similar efficacy and side effects to Wegovy.
None of these medications is objectively "safest" — safety depends on your individual circumstances. A prescriber will assess your medical history, current medications, and health conditions to determine which is most appropriate for you. Use our free GLP-1 weight loss calculator to explore how different medications might work based on the clinical trial data.
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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.
Finding out if you're eligible and next steps with your GP
To find out which injection might be safe and suitable for you, start by discussing weight management options with your GP. You may be eligible for prescription weight loss treatment if you have a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. Your GP can refer you to a specialist weight management service (available through NHS tier 3 services) or discuss private prescribing options.
Check your personal BMI using our free BMI calculator to understand where you stand. Access via NHS specialist services can involve waiting lists, but private online prescribing is also available through registered UK prescribers — all require clinical assessment before dispensing. The safest choice is always the one discussed and approved by a healthcare professional who knows your full medical history.
- 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.
Are weight loss injections safe for everyone?
Weight loss injections are prescription-only and not suitable for everyone. They're contraindicated in pregnancy, breastfeeding, and certain medical conditions. Eligibility is assessed individually by a prescriber based on your health profile, medications, and BMI. Always inform your healthcare provider of any existing health conditions before starting treatment.
Which weight loss injection has the fewest side effects?
All three UK-licensed injections (Mounjaro, Wegovy, Saxenda) have similar side effect profiles, with nausea and digestive changes most common. Severity varies between individuals and often improves over time. There's no reliable evidence that one has significantly fewer side effects than another — individual tolerance varies considerably.
Can I switch between Mounjaro and Wegovy if I don't tolerate one?
Switching between medications should only be done under medical supervision. A prescriber may suggest switching if you experience intolerable side effects, but this requires a new clinical assessment and must be managed carefully to ensure safe dosing transitions.
What makes a weight loss injection "safe"?
Safety depends on MHRA approval (clinical trial evidence), individual health factors, appropriate dosing, correct injection technique, and medical supervision. It also means choosing one that's compatible with your other medications and medical history — why clinical assessment before prescription is essential.
How do I get a weight loss injection safely in the UK?
Always obtain prescription weight loss injections through either NHS specialist weight management services (via GP referral) or from a GPhC-registered pharmacy following a clinical assessment by a UK-licensed prescriber. Never buy from unregistered sources. Both pathways involve proper medical evaluation to ensure safety.
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.