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 Injection Side Effects — What to Expect
Weight loss injections are prescription medications used for weight management in the UK, but they do come with potential side effects. The most commonly prescribed options — Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide) — work by reducing appetite and helping you feel fuller after meals. Understanding what side effects may occur, how long they typically last, and when to seek medical help is essential before starting treatment. This guide covers what you need to know about weight loss injection side effects in a UK health context.
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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
Most side effects associated with prescription weight loss injections are gastrointestinal (stomach and digestion-related). The most frequently reported side effects include nausea, diarrhoea, vomiting, and constipation. These typically occur in the first few weeks of treatment as your body adjusts to the medication.
Very common side effects (affecting more than 1 in 10 people) include:
- Nausea and vomiting
- Diarrhoea or constipation
- Stomach pain or indigestion
- Decreased appetite
- Fatigue or dizziness
Less common but possible side effects include injection site reactions (redness or bruising), headache, and hair loss. Side effects are often dose-dependent — meaning they may be more pronounced when starting or when increasing your dose. Most people find that gastrointestinal symptoms improve significantly within 2–4 weeks as their body adjusts. Your GP or prescriber can advise on managing symptoms, such as adjusting your diet or staying hydrated.
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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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NHS-Approved Weight Loss Options and Eligibility
In the UK, three prescription medications are licensed for weight management: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All require a clinical assessment before prescription. You may be eligible if you have a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea.
Access to these medications in the UK is through either NHS specialist weight management services (via GP referral) or private online prescribing. NHS access varies significantly by region and can involve waiting lists. Your GP can discuss whether prescription weight loss medication is appropriate for you and what side effects to expect. Alongside medication, sustainable weight loss requires dietary changes and increased physical activity for best results.
- 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 injections?
The most common side effects are gastrointestinal: nausea, diarrhoea, vomiting, constipation, and stomach pain. These typically affect more than 1 in 10 people and usually improve within 2–4 weeks as your body adjusts. Less common side effects include fatigue, dizziness, headache, and injection site reactions.
How long do weight loss injection side effects last?
Most side effects peak in the first 1–2 weeks and gradually improve over 2–4 weeks. However, some people experience nausea or stomach discomfort for longer. If side effects persist beyond 4 weeks or are severe, contact your GP or prescriber — they may adjust your dose or suggest dietary changes to help manage symptoms.
When should I seek medical attention for weight loss injection side effects?
Seek urgent medical help if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face or throat), or changes in vision. Contact your GP if you have severe or prolonged vomiting, signs of dehydration, or side effects that significantly affect your daily life.
Can I reduce side effects by changing my diet?
Yes. Many people find that eating smaller, more frequent meals, avoiding fatty or greasy foods, and staying well-hydrated helps reduce nausea and digestive symptoms. Your GP or a dietitian can provide personalised dietary advice. Never stop or reduce your dose without medical guidance — always speak to your prescriber first.
Do all weight loss injections have the same side effects?
All three UK-licensed options (Mounjaro, Wegovy, and Saxenda) cause similar gastrointestinal side effects because they work through similar mechanisms. However, individual responses vary — some people experience mild side effects whilst others experience more pronounced symptoms. Your prescriber can discuss which option may be most suitable for you based on your health history.
Ready to take control of your weight loss journey?
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