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 Fat Jabs Safe? Understanding UK Weight Loss Injections
Yes, fat jabs (weight loss injections) are safe when prescribed and monitored by a healthcare professional in the UK. "Fat jabs" is a colloquial term for prescription-only GLP-1 and GIP/GLP-1 receptor agonist medications licensed for weight management. All UK-approved weight loss injections have undergone rigorous safety testing and clinical trials. However, like all medications, they have side effects and eligibility requirements. This guide compares the three licensed options available in the UK and explains how to access them safely through NHS or private prescribing.
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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 Injection Options Compared
The UK has three prescription-only weight loss injections currently licensed for adults: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Each works through different mechanisms and dosing schedules.
Mounjaro is a once-weekly injection combining GIP and GLP-1 receptor agonist action. Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks at maximum dose. Wegovy is a once-weekly GLP-1 injection with average weight loss of approximately 15% over 68 weeks. Saxenda is a daily GLP-1 injection with more modest average weight loss around 5–10%.
All three are prescription-only and require a clinical assessment to confirm eligibility. Safety depends on proper medical monitoring, appropriate dose escalation (typically 4 weeks between increases), and disclosure of your full medical history to your prescriber. Side effects are common in the first weeks—nausea, diarrhoea, constipation, and stomach pain—but typically improve with time or dose adjustment.
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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.
NHS vs Private Access: Getting a Safe Prescription
In the UK, weight loss injections are available through two routes. NHS access is through specialist tier 3 weight management services, usually requiring a GP referral. Availability varies significantly by region and waiting lists can be substantial. Private access is through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber—no NHS referral needed.
Both routes require eligibility assessment: typically a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, hypertension, or sleep apnoea. Your prescriber will review your medical history, current medications, and contraindications before issuing a prescription. Use our free BMI calculator to check where you stand, and discuss weight loss options with your GP to explore both NHS and private pathways suited to your circumstances.
- 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 makes UK fat jabs safe compared to unregulated options?
UK-licensed weight loss injections are approved by the MHRA, have passed rigorous clinical trials, and come with full safety information and monitoring requirements. They're only available by prescription after clinical assessment. Unregulated or counterfeit injections from unlicensed sources carry serious risks of contamination, incorrect dosing, and no medical oversight—never buy weight loss injections without a valid UK prescription.
Which fat jab is safest—Mounjaro, Wegovy, or Saxenda?
All three are safe when used as prescribed under clinical supervision. Safety isn't about which is "safest"—it's about which is right for you. Mounjaro and Wegovy are more potent (higher average weight loss) and both have more research data. Saxenda is a daily injection with a longer track record. Your prescriber will recommend based on your health, medications, and medical history.
Can I use fat jabs without seeing a doctor?
No. All UK-licensed weight loss injections are prescription-only medicines. You must have a clinical assessment—either with your NHS GP or a private UK-licensed prescriber via an online consultation—before a valid prescription can be issued. This assessment protects your safety by checking for contraindications, drug interactions, and ensuring the medication is appropriate for you.
What are the most common side effects of fat jabs?
Nausea, diarrhoea, constipation, vomiting, and stomach pain are very common, especially in the first 4 weeks. These often improve with time. Less common: headache, fatigue, dizziness, injection site reactions, and hair loss. Serious side effects are rare but include pancreatitis signs (severe stomach pain to the back) and gallbladder problems—seek immediate medical attention if these occur.
Will I need blood tests or monitoring while using fat jabs?
Yes. Your prescriber will typically recommend baseline blood tests before starting and periodic check-ins while on the medication, especially if you have diabetes. You should report any concerning symptoms to your healthcare provider. Regular monitoring ensures the medication is working safely and any side effects are managed appropriately.
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.