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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Weight Loss Jab Side Effects in the UK
Weight loss jabs — prescription injections for weight management — have become increasingly available in the UK through both NHS specialist services and private prescription options. If you're considering this form of weight management in the UK, understanding the possible side effects is essential before starting treatment. This guide covers what these medications are, who is eligible, and what side effects you should know about before speaking to your GP.
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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 Jabs and Their Side Effects
Three prescription weight loss jabs are currently licensed for use in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Mounjaro and Wegovy are once-weekly injections; Saxenda is a daily injection. Common side effects across all three include nausea, diarrhoea, constipation, vomiting, and stomach pain — these tend to be more pronounced when starting treatment or increasing doses.
- Mounjaro: Very common side effects affect more than 1 in 10 people and include nausea, diarrhoea, vomiting, constipation, and abdominal pain. Most improve within the first few weeks as your body adjusts.
- Wegovy: Similar profile to Mounjaro, with nausea and diarrhoea being most common. Also common are headache, fatigue, and dizziness.
- Saxenda: Gastrointestinal side effects dominate, with nausea reported in most users during the initial phase.
In clinical trials, side effects were typically mild to moderate and decreased over time. However, individual experiences vary significantly — some people experience minimal side effects, whilst others find them more troublesome.
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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.
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Accessing Weight Loss Jabs: NHS and Private Routes in the UK
In the UK, prescription weight loss jabs are available through two main pathways. NHS access requires referral to a specialist tier 3 or tier 4 weight management service — typically from your GP. Availability varies significantly by region and waiting lists can be lengthy. Private access is available through GPhC-registered online prescribers following an online clinical assessment, usually without an NHS referral.
Eligibility for weight management medication in the UK includes: BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean backgrounds, lower BMI thresholds may apply. To understand whether you're eligible and which medication might suit you best, start a conversation with your GP or a licensed prescriber — they can assess your individual situation and discuss risks and benefits with you.
---COL_J_TOOL_REFERENCE--- Use our free BMI calculator above to check your personal BMI instantly and see which category you fall into. Our GLP-1 weight loss calculator can also help you understand the potential outcomes alongside these medications.- 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 in the UK?
The most common side effects are nausea, diarrhoea, constipation, vomiting, and stomach pain. These gastrointestinal effects are very common when starting treatment or increasing doses, but typically improve within a few weeks as your body adapts. Other common side effects include fatigue, headache, dizziness, and injection site reactions. Most people find side effects manageable and they decrease over time.
Should I stop taking my weight loss jab if I experience side effects?
No — do not stop without speaking to your prescriber first. Many side effects improve naturally within a few weeks. Your prescriber may suggest taking the injection at a different time of day, adjusting your diet, or staying on a dose longer before increasing it. Serious side effects like persistent severe stomach pain, gallbladder problems, or allergic reactions do require immediate medical attention and you should contact your GP or NHS 111 urgently.
Are weight loss jabs safe to use long-term in the UK?
Current evidence suggests weight loss jabs are safe for long-term use when prescribed and monitored appropriately. UK clinical trials have followed users for up to 68 weeks with good safety profiles. However, long-term safety data beyond two years is still emerging. Your GP or prescriber will monitor your health regularly if you're on a weight loss jab and discuss any concerns with you.
Can I get a weight loss jab on the NHS in the UK?
Yes, weight loss jabs are available on the NHS through specialist weight management services for eligible adults. However, access varies significantly by region and waiting times can be lengthy. You'll need a referral from your GP to a tier 3 or tier 4 specialist service. Private access is also available through online prescribers without an NHS referral, though this involves a cost to you.
What's the difference between Mounjaro, Wegovy, and Saxenda for side effects?
All three cause similar gastrointestinal side effects (nausea, diarrhoea, constipation, stomach pain), but the timing and frequency differ. Mounjaro and Wegovy are once-weekly injections, whilst Saxenda is a daily injection. Mounjaro is a dual GIP/GLP-1 medication and Wegovy is a GLP-1 medication — individual side effect experiences vary. Your prescriber will help choose which is most suitable for you based on your health profile and preferences.
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