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 Injections UK Side Effects — Complete Guide
Weight loss injections available in the UK, including Wegovy (semaglutide) and Mounjaro (tirzepatide), are prescription-only GLP-1 medications that can help with weight management. Like all medications, they come with potential side effects. Understanding what to expect helps you make an informed decision about whether prescription weight loss medication is right for you. This guide covers the most common side effects, how long they typically last, and when to seek medical help. Always consult your GP before starting any weight management programme.
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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.
Common Side Effects of Weight Loss Injections
Weight loss injections work by mimicking GLP-1, a natural hormone that affects appetite and blood sugar. The most frequently reported side effects are gastrointestinal (digestive) — affecting how your stomach and intestines work. These are particularly common when you first start the medication or increase your dose.
Very common side effects (affecting more than 1 in 10 people):
- Nausea — often mild and improves over time
- Diarrhoea or constipation — changes in bowel habits
- Vomiting — less common than nausea
- Stomach pain or indigestion
- Loss of appetite — this is partly how the medication works
Most people find these side effects settle within the first few weeks as their body adjusts. Eating smaller, lighter meals and staying hydrated can help. If side effects are severe or persistent, speak to your prescriber — they may suggest delaying your next dose increase or adjusting your approach.
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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.
Less Common But Important Side Effects
Common side effects (affecting up to 1 in 10 people): headaches, dizziness, fatigue, injection site reactions (redness or bruising), and hair loss. Some people also report changes in taste or heartburn. These are generally mild and temporary.
Serious side effects (rare but require immediate medical attention): severe stomach pain spreading to your back (possible pancreatitis), gallbladder problems, serious allergic reactions, or changes in vision. If you experience any of these, stop the injection and seek urgent medical help immediately. Always inform your GP of all existing health conditions before starting weight loss medication, as some conditions may increase your risk of certain side effects.
- 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.
How long do weight loss injection side effects last?
Most side effects are worst in the first few days after injection and typically improve within 1–2 weeks. Nausea and digestive issues usually settle as your body adjusts, though some people experience them for several weeks. Dose increases can temporarily bring side effects back. Most side effects are mild and manageable, but speak to your prescriber if they're severe or persistent.
Are weight loss injections safer than other weight management methods?
Weight loss injections are prescription-only medicines approved by the MHRA for weight management in UK adults. Clinical trials (STEP for Wegovy, SURMOUNT for Mounjaro) show they're effective and side effects are generally manageable. However, all medications carry risks. Your GP will assess whether they're appropriate for you based on your health history and any existing conditions.
Can I manage side effects without stopping the injection?
Yes, in most cases. Eating smaller, lighter meals and staying hydrated helps with nausea and digestive issues. Ginger, peppermint tea, and avoiding fatty foods may help. However, if side effects are severe or you develop signs of pancreatitis (severe stomach pain), stop the injection and contact your GP immediately. Never adjust your dose without speaking to your prescriber.
What's the difference between Wegovy and Mounjaro side effects?
Both are GLP-1-based medications with similar side effect profiles — nausea, diarrhoea, and stomach pain are most common. Mounjaro (dual GIP/GLP-1) and Wegovy (GLP-1 only) work slightly differently, so some people tolerate one better than the other. Individual responses vary. Your prescriber can discuss which might suit you best.
Should I use our GLP-1 weight loss calculator if I'm worried about side effects?
Yes. Our free weight loss calculator can help you understand projected weight loss outcomes at different doses alongside your prescriber's guidance. Seeing realistic expectations — and knowing that average weight loss is 15–21% of body weight — may help you weigh the benefits against side effects when deciding if medication is right for you.
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.