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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Fat Loss Injections Side Effects — What You Need to Know
Fat loss injections are prescription-only weight loss medications available through UK healthcare. These GLP-1 and GLP-1/GIP receptor agonist medications can help support weight management alongside diet and exercise. Three main options are licensed in the UK: Mounjaro, Wegovy, and Saxenda. All carry potential side effects that vary between individuals. Understanding your options and what to expect is essential before starting any prescription slimming injections. This guide covers all UK-licensed options, their mechanisms, and how to access them through NHS or private routes.
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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.
UK-Licensed Weight Loss Injections — Side Effects and Comparison
Three weight loss injection options are currently licensed in the UK for adults with obesity or overweight with health conditions:
- Mounjaro (tirzepatide) — dual GIP and GLP-1 receptor agonist, once weekly, doses 2.5mg to 15mg. Clinical trials showed average weight loss of 15–21% body weight over 72 weeks.
- Wegovy (semaglutide) — GLP-1 receptor agonist only, once weekly, doses 0.25mg to 2.4mg. Average weight loss approximately 15% body weight in clinical trials.
- Saxenda (liraglutide) — GLP-1 receptor agonist, once daily injection, doses 0.6mg to 3mg. Modest weight loss outcomes, typically 5–10% body weight.
All three work by reducing appetite and supporting fullness signals, though Mounjaro's dual mechanism may offer different results. All are prescription-only requiring clinical assessment. Side effects are broadly similar across all three: nausea, diarrhoea, vomiting, and constipation are very common, especially during dose escalation. Individual tolerance varies significantly.
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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 Fat Loss Injections — NHS vs Private in the UK
In the UK, weight loss injections are accessed through two main routes. NHS access requires referral to a tier 3 specialist weight management service through your GP. NICE has approved tirzepatide (Mounjaro) for use within NHS services, though availability varies significantly by region and waiting lists can be lengthy. Private access is available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber — no NHS referral needed. Eligibility criteria are the same for both routes: BMI 30 or above, or BMI 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. Use our free GLP-1 weight loss calculator to estimate potential results, and our BMI calculator to check your starting point.
- 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?
Nausea, diarrhoea, vomiting, and constipation are very common, particularly during the first few weeks and when doses increase. Most people find these settle within 2–4 weeks as their body adjusts. Other common side effects include indigestion, fatigue, and injection site reactions. Individual experiences vary widely — some people experience minimal side effects whilst others find them more pronounced.
Which weight loss injection has the fewest side effects?
Side effect profiles are broadly similar across Mounjaro, Wegovy, and Saxenda. All GLP-1 medications cause similar gastrointestinal effects. Individual tolerance is highly variable — what causes problems for one person may cause none for another. Your prescriber will help monitor side effects and adjust your dose if needed.
Can I switch between different weight loss injections if I experience side effects?
Possibly, but this requires prescriber guidance. Switching between Mounjaro, Wegovy, and Saxenda is not straightforward — they have different mechanisms and dosing schedules. If side effects are problematic, your prescriber may suggest dose reduction, slower escalation, or dietary adjustments before considering a switch to a different medication.
How long do side effects of weight loss injections last?
Most people find gastrointestinal side effects reduce within 2–4 weeks of starting or increasing a dose. However, some individuals experience longer adjustment periods. Maintaining hydration, eating smaller meals, and avoiding fatty foods can help. If side effects persist beyond 4–6 weeks, contact your prescriber for guidance.
Are weight loss injections safe long-term?
Clinical trials supporting Mounjaro and Wegovy ran for 68–72 weeks, showing sustained safety in that timeframe. Long-term safety data (beyond 2–3 years) is still being collected. All prescription weight loss medications carry potential risks and require ongoing prescriber monitoring. Your GP or prescriber will assess whether this treatment is appropriate for your individual health profile.
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.