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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Slimming Injection Side Effects — What UK Adults Need to Know
Slimming injections are prescription-only weight loss medications licensed in the UK, and side effects vary depending on which medication you're prescribed. The most commonly used prescription slimming injections — Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide) — all work as GLP-1 or dual GIP/GLP-1 receptor agonists. Understanding potential side effects, how they compare, and which option might suit you is essential before starting treatment. This guide covers the facts about side effects across all UK-licensed options, plus eligibility and how to access them through the NHS or privately.
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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-Licensed Slimming Injections
All three UK-licensed weight loss injections share similar common side effects because they work on similar hormone pathways, though the intensity and frequency vary between individuals:
- Mounjaro (tirzepatide) — dual GIP and GLP-1 agonist, once-weekly injection. Very common side effects: nausea, diarrhoea, vomiting, constipation, stomach pain. Clinical trials showed approximately 15–21% average body weight loss at maximum dose (15 mg).
- Wegovy (semaglutide) — GLP-1 agonist only, once-weekly injection. Very common side effects: nausea, diarrhoea, vomiting, constipation, stomach pain. Clinical trials showed approximately 15% average body weight loss at maximum dose (2.4 mg).
- Saxenda (liraglutide) — GLP-1 agonist, daily injection. Similar common side effects but delivered daily rather than weekly. Average weight loss approximately 5–7% body weight.
Most side effects are mild to moderate and tend to improve over time as your body adjusts. Serious side effects are rare but require urgent medical attention: severe stomach pain (possible pancreatitis), allergic reactions, or gallbladder complications.
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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.
Accessing Slimming Injections in the UK — NHS vs Private
Eligibility for prescription slimming injections is the same whether you pursue NHS or private access: BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, sleep apnoea, cardiovascular disease). Lower thresholds apply if you're of South Asian, Chinese, Black African, or Black Caribbean heritage.
NHS access: Referral through your GP to tier 3 specialist weight management services. Limited availability and potentially long waiting lists depending on your region. Private access: Online consultation with a UK-licensed prescriber, prescription issued if clinically appropriate, dispensed by a GPhC-registered pharmacy. Available immediately without NHS referral. Your GP can advise which option suits you, and a prescriber will assess whether side effects from a particular medication might affect your health or other 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.
Which UK slimming injection has the fewest side effects?
Side effect profiles are similar across all three UK-licensed injections because they work on similar hormone pathways. Individual tolerance varies — some people experience minimal side effects, others notice more. Your prescriber will assess your medical history to recommend the best option. Most side effects improve as your body adjusts to the medication over the first few weeks.
Do slimming injection side effects go away?
Yes, typically. Most side effects like nausea, constipation, and diarrhoea are most pronounced in the first 4 weeks and often improve as your body adjusts. However, some people continue to experience mild side effects throughout treatment. Staying hydrated, eating smaller meals, and eating fibre-rich foods can help manage gastrointestinal side effects.
What's the difference between Mounjaro and Wegovy side effects?
Both have similar very common side effects (nausea, diarrhoea, vomiting, constipation, stomach pain). Mounjaro is a dual GIP/GLP-1 agonist while Wegovy is GLP-1 only — this means Mounjaro may work differently in your body. Individual experiences vary significantly. Your prescriber will discuss which medication is more appropriate based on your health, medications, and preferences.
Can I stop taking slimming injections if side effects are severe?
Yes. If you experience severe or intolerable side effects, contact your prescriber immediately. You can stop the medication, and your prescriber may suggest a different option or dose adjustment. Always speak to your healthcare professional before stopping — do not stop abruptly. Keep track of side effects using our free progress tracker to share with your prescriber.
Are slimming injections safe long-term?
UK-licensed slimming injections (Mounjaro, Wegovy, Saxenda) have undergone rigorous safety testing and are monitored by the MHRA. Long-term safety data supports their use in adults who meet eligibility criteria. However, they're prescription-only medicines requiring ongoing medical supervision. Your GP or prescriber will monitor your health regularly and can answer specific safety questions about your individual circumstances.
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.