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 injections Side Effects — What You Need to Know
"weight loss injections" is a colloquial UK term for prescription weight loss injections like Wegovy (semaglutide) and Mounjaro (tirzepatide). While these GLP-1 medications can support weight management, they do come with potential side effects. Understanding what these are, how common they are, and when to seek medical advice is essential for anyone considering this treatment. This UK health guide explains the most frequently reported side effects and what NHS guidance recommends.
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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
The most frequently reported side effects of GLP-1 weight loss injections like Wegovy and Mounjaro are gastrointestinal — affecting the stomach and digestive system. In clinical trials, these were very common, meaning they affected more than 1 in 10 people.
- Nausea — the most common side effect; typically improves over time as your body adjusts
- Diarrhoea — affects digestion; staying hydrated and adjusting diet can help
- Constipation — paradoxically, some people experience this instead of diarrhoea
- Vomiting — less common but possible, especially when starting or increasing dose
- Stomach pain — mild to moderate discomfort in the abdomen
- Indigestion — acid reflux or heartburn symptoms
These effects often decrease as your body adjusts to the medication, typically within the first few weeks. Using our free BMI calculator can help you determine if weight loss treatment might be appropriate for your health profile before discussing with your GP.
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How It Works
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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 Help
While uncommon, some serious side effects require immediate medical attention. Seek urgent care if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, or throat), or sudden changes in vision. Gallbladder problems, rapid or irregular heartbeat, and severe dehydration from vomiting or diarrhoea are also reasons to contact your GP or NHS 111 immediately.
Other common but less severe effects include fatigue, dizziness, headache, hair loss, and injection site reactions (redness or bruising). These are usually manageable and temporary. Always inform your prescriber of any side effects during your follow-up appointments. Your GP or the prescriber can adjust your dose, suggest management strategies, or recommend stopping the medication if side effects are intolerable.
- 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 weight loss injections side effects last?
Most gastrointestinal side effects improve within the first 2–4 weeks as your body adjusts. Nausea typically decreases significantly by week 3–4. However, some people may experience side effects longer. If they persist beyond 8 weeks, discuss with your prescriber — they may adjust your dose or explore management strategies. Individual experiences vary widely.
Are weight loss injections side effects worse when starting or increasing dose?
Yes. Side effects are typically most noticeable when you start the medication or when your dose increases. This is why weight loss injections use a gradual escalation schedule — starting at a low dose and increasing every 4 weeks. This allows your body to adjust gradually. Side effects usually settle as your body acclimates to the higher dose.
Can you prevent weight loss injections side effects?
You cannot completely prevent them, but several strategies may help: eat smaller, frequent meals; avoid fatty or spicy foods; stay well hydrated; take the injection on the same day each week; and follow any dietary advice from your prescriber or a dietitian. Using our GLP-1 weight loss calculator and discussing realistic expectations with your GP before starting can also prepare you mentally for potential side effects.
What's the difference between Wegovy and Mounjaro side effects?
Both are GLP-1-based injections with very similar side effect profiles. Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. In practice, the side effects are comparable — nausea, diarrhoea, constipation, and stomach pain are common with both. Individual tolerance varies; some people tolerate one better than the other.
Should I stop taking the injection if I have side effects?
Do not stop without consulting your prescriber. Many side effects improve with time and dose adjustment. Your GP or prescriber may suggest temporarily staying at your current dose longer, switching to a different medication, or implementing management strategies. Stopping abruptly without guidance is not recommended — discuss any concerns with your healthcare provider during a follow-up appointment.
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