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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Side Effects of Slimming Injections: UK Options Explained
Slimming injections are prescription-only medications used to support weight loss in UK adults. The three main weight loss injections available are Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All three work as GLP-1 or dual GIP/GLP-1 receptor agonists and share similar side effect profiles, though individual experiences vary. Understanding the potential side effects is essential before starting any prescription slimming injections, whether through NHS specialist services or private prescribing. This guide compares all UK options and explains what you might expect.
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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.
Side Effects Across UK Slimming Injections
All three UK-licensed weight loss injections share a common mechanism and therefore have overlapping side effect profiles. Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. Most people experience these when starting or increasing doses, and they typically improve within 2–4 weeks as the body adjusts.
Common side effects (up to 1 in 10 people) include indigestion, acid reflux, injection site reactions, fatigue, dizziness, and in some cases, hair loss. Serious side effects are rare but require immediate medical attention: severe stomach pain spreading to the back (possible pancreatitis), gallbladder problems, serious allergic reactions, or unusual changes in heart rate.
- Mounjaro (tirzepatide): Dual GIP+GLP-1 agonist, once-weekly injection, doses 2.5–15 mg
- Wegovy (semaglutide): GLP-1 agonist, once-weekly injection, doses 0.25–2.4 mg
- Saxenda (liraglutide): GLP-1 agonist, once-daily injection, doses 0.6–3 mg
Side effect severity often correlates with dose: lower starting doses produce fewer symptoms, and escalating gradually reduces discomfort. Your prescriber will adjust your dose based on tolerability.
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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 and Eligibility
In the UK, you can access prescription slimming injections through the NHS or privately. NHS access is via specialist tier 3/4 weight management services—typically requiring a GP referral and assessment of eligibility. NHS availability varies by region, and waiting lists can be lengthy. Private access is through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber; no NHS referral is required.
Eligibility criteria are consistent across both routes: adults with a BMI of 30 kg/m² or above, or BMI of 27–29.9 kg/m² with weight-related health conditions (type 2 diabetes, hypertension, dyslipidaemia, sleep apnoea, or cardiovascular disease). Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may qualify at lower BMI thresholds. Pregnant women, those planning pregnancy, and anyone under 18 cannot use these medications. Use our free BMI calculator to check your starting point, and discuss which access route suits your circumstances with your GP or prescriber.
- 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.
Are side effects the same for all three weight loss injections?
Largely yes—all three are GLP-1 or GIP/GLP-1 agonists sharing similar side effect profiles. The most common are nausea, diarrhoea, constipation, and stomach pain, especially when starting or increasing doses. Individual tolerance varies, and your prescriber may switch medications if side effects are problematic.
How long do side effects from slimming injections last?
Most side effects improve within 2–4 weeks as your body adapts to the medication. Nausea and digestive issues are typically worst in the first few days after each injection. If side effects persist beyond 4 weeks, discuss them with your prescriber—dose adjustments or alternative medications may help.
Can I take slimming injections if I have type 2 diabetes?
Yes, many people with type 2 diabetes take weight loss injections. However, if you're also taking diabetes medications like insulin or sulphonylureas, your prescriber may need to adjust those doses. Always inform your prescriber of all medications you're taking.
What's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist; Wegovy (semaglutide) is a GLP-1-only agonist. Clinical trials suggest Mounjaro may produce slightly greater average weight loss (~15–21% vs ~15% body weight), but results vary between individuals. Both require prescription and clinical assessment. Your prescriber will advise which suits you best.
Can I access slimming injections on the NHS in my area?
NHS access depends on your local ICB (Integrated Care Board) and tier 3/4 weight management service availability. Some regions have longer waiting lists than others. Contact your GP to enquire about local NHS options, or ask about private prescribing through a UK-licensed online service if NHS access is limited.
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.