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
Which weight loss injection is safest?
Safety depends on your individual health profile, not the medication alone. The UK-licensed weight loss injections—Wegovy (semaglutide), Mounjaro (tirzepatide), and Saxenda (liraglutide)—are all prescription-only medicines approved by the MHRA and undergo strict safety monitoring. All three have been used successfully in clinical trials, though side effects, eligibility, and suitability vary between individuals. Your GP or a licensed prescriber will assess which option is safest for you based on your health history, current medications, and weight management goals.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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
Three weight loss injections are currently licensed in the UK for weight management:
- Wegovy (semaglutide) — GLP-1 receptor agonist, once-weekly injection, maximum dose 2.4 mg. Clinical trials (STEP programme) showed average weight loss of around 15% body weight. Common side effects include nausea, diarrhoea, and constipation, typically mild and transient.
- Mounjaro (tirzepatide) — Dual GIP/GLP-1 receptor agonist, once-weekly injection, maximum dose 15 mg. SURMOUNT trials showed average weight loss of 15–21% body weight. Similar side effect profile to Wegovy but may have different tolerability for individual patients.
- Saxenda (liraglutide) — GLP-1 receptor agonist, daily injection, maximum dose 3 mg. Longer-established in the UK. All three are prescription-only and require clinical assessment before prescribing. Safety is monitored through MHRA's Black Triangle (▼) scheme for newer medications.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Assessing safety for your individual circumstances
No single weight loss injection is "safest" universally—safety depends on your medical history, current medications, and health conditions. Your GP or licensed prescriber will review: any history of thyroid cancer or MTC, kidney or liver function, diabetes medications (which may need adjustment), and whether you're taking oral contraceptives (absorption may be affected). Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, then discuss results with your prescriber. Weight loss injections work best alongside dietary changes and increased physical activity, not as standalone treatment.
- 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.
Is Wegovy safer than Mounjaro?
Both are MHRA-approved and have undergone rigorous safety testing. Wegovy has longer real-world use data in the UK. Mounjaro is newer but shows comparable safety in clinical trials. Individual safety depends on your health profile, not the medication brand. Your prescriber will recommend the safest option for you.
What are the most common side effects of weight loss injections?
Nausea, diarrhoea, vomiting, and constipation are very common, affecting more than 1 in 10 users. These are usually mild and improve over time as your body adjusts. Inject site reactions (redness, bruising) are also common. Serious side effects are rare but require immediate medical attention.
Can I switch between weight loss injections if one doesn't suit me?
Yes, switching is possible under prescriber guidance. You cannot simply swap doses between medications—your prescriber will determine the appropriate transition. Any switch requires clinical reassessment. Some people tolerate one medication better than another.
How do I access the safest weight loss injection for me in the NHS?
Refer to your GP for assessment and potential referral to a tier 3 specialist weight management service. NHS access is available but varies by region and wait times can be lengthy. Private prescribing is faster but requires payment.
Are there any serious safety concerns with weight loss injections?
Serious side effects are rare but include pancreatitis (severe stomach pain), gallbladder problems, and severe allergic reactions. These require immediate medical attention. MHRA continues to monitor safety—inform your prescriber of any unusual symptoms, and never exceed prescribed doses.
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.