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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Retatrutide Weight Loss Side Effects — What to Expect
Retatrutide is a dual GIP and GLP-1 receptor agonist injection approved in the UK for weight management in adults. Like all prescription weight loss medications, it can cause side effects — some common, others rare. Understanding what to expect helps you make an informed decision with your GP or prescriber. This guide covers the most frequently reported side effects, when they typically occur, and what action to take if you experience concerning symptoms. Always consult your healthcare professional before starting any weight management medication.
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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 Retatrutide
Retatrutide commonly causes gastrointestinal side effects, especially during the dose escalation phase (the first 4–8 weeks). Most side effects are mild to moderate and often improve as your body adjusts.
- Nausea: Very common in the first 2–4 weeks; usually lessens over time
- Diarrhoea: Affects many users during dose increases; typically improves within days to weeks
- Constipation: Can alternate with diarrhoea as your gut adapts
- Stomach pain or discomfort: Mild cramping is common; severe pain requires medical attention
- Vomiting: Less common than nausea but can occur; report if persistent
- Indigestion: Heartburn and acid reflux may develop, especially with larger meals
- Fatigue: Some users report tiredness, particularly in the first 1–2 weeks
- Headache: Mild to moderate headaches are reported by a minority of users
- Dizziness: May occur, particularly when standing up quickly
- Appetite loss: Expected effect, but sometimes stronger than anticipated
These side effects are not dangerous but can be uncomfortable. Most resolve within 4–8 weeks as your body acclimates to the medication.
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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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Serious Side Effects — When to Seek Medical Attention
Serious side effects are rare but require immediate medical attention. Seek urgent help if you experience severe stomach pain spreading to your back (possible pancreatitis), persistent vomiting lasting more than a few hours, signs of a serious allergic reaction (swelling of face, lips, tongue, or throat), changes in vision or vision problems, rapid heartbeat or chest pain, or signs of gallbladder problems (pain in upper abdomen, yellowing of skin or eyes).
Contact your GP or NHS 111 immediately if you have concerning symptoms. Never ignore persistent or worsening side effects. Your prescriber can adjust your dose, change your medication, or provide additional support based on your individual response.
- 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 retatrutide side effects last?
Most common side effects peak during dose increases and improve within 4–8 weeks as your body adapts. Nausea and diarrhoea typically resolve first. Some users experience mild side effects throughout treatment; others experience none. Individual variation is significant. Always discuss ongoing side effects with your prescriber.
Can I reduce side effects without stopping retatrutide?
Yes. Slow your dose escalation, eat smaller frequent meals, stay hydrated, and avoid fatty or greasy foods. Your prescriber can slow your dose schedule or recommend anti-nausea strategies. Some side effects improve with dietary adjustments. Paracetamol can help with headaches; speak to your pharmacist about safe options alongside retatrutide.
Are retatrutide side effects the same as Mounjaro or Wegovy?
Retatrutide, Mounjaro (tirzepatide), and Wegovy (semaglutide) all cause similar GI side effects because they work on related hormone pathways. The pattern and severity vary between individuals. Some people tolerate one medication better than another. Your prescriber can help you find the best option if you experience intolerable side effects.
What's the difference between expected side effects and something serious?
Expected side effects (nausea, diarrhoea, mild stomach discomfort) are uncomfortable but not dangerous and improve over time. Serious side effects (severe stomach pain, persistent vomiting, allergic reactions, vision changes, chest pain) require urgent medical attention. When in doubt, contact your GP or NHS 111 — it's always better to check.
Should I stop retatrutide if I get side effects?
Do not stop without medical guidance. Most side effects resolve with time and dietary adjustments. Contact your prescriber first — they may slow your dose escalation, recommend management strategies, or switch to a different weight management medication. Your GP or prescriber will help you weigh the benefits against side effects for your individual health.
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.