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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Semaglutide Weight Loss Side Effects — What to Expect
Semaglutide is a prescription-only GLP-1 receptor agonist used for weight loss in the UK under the brand name Wegovy. Like all medications, it can cause side effects — though many are temporary and improve over time. Understanding what to expect helps you make an informed decision about whether semaglutide weight loss treatment is right for you. This guide covers the most common side effects, how long they typically last, and when you should contact your GP.
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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 Semaglutide Side Effects and What to Expect
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These are usually mild to moderate and often improve within the first few weeks as your body adjusts to the weekly weight loss injection.
Nausea is the most frequently reported side effect. It tends to be worse on the day of injection or the day after, and typically decreases over time as the dose escalates gradually during the initial 4–5 months. Most people report that nausea becomes manageable or resolves entirely by week 8–12.
Common side effects (affecting up to 1 in 10 people) include indigestion, headache, fatigue, dizziness, hair loss, and injection site reactions (redness or bruising). Eating smaller, lighter meals and staying hydrated can help reduce gastrointestinal symptoms. If side effects persist beyond 8 weeks, contact your GP or prescriber for guidance.
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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.
Serious Side Effects and When to Seek Help
While serious side effects are rare, seek immediate medical attention if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, or throat), changes in vision, or signs of gallbladder problems (pain in upper stomach or right side). These require urgent assessment.
Contact your GP if side effects do not improve after 2–3 weeks, worsen over time, or significantly affect your daily life. Your prescriber may adjust your dose, suggest timing changes, or discuss alternative treatments. Keep a symptom diary to track patterns — this helps your healthcare provider make personalised recommendations.
- 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 semaglutide side effects last?
Most side effects like nausea and diarrhoea improve within 2–4 weeks as your body adjusts. By week 8–12, many people report significant improvement. Side effects may reappear temporarily when your dose increases, but typically resolve within 1–2 weeks. Everyone responds differently, so speak to your GP if symptoms persist beyond this timeframe.
Is nausea the most common side effect of semaglutide?
Yes, nausea is the most frequently reported side effect of semaglutide weight loss treatment. It's usually mild to moderate and often improves within the first month. Eating smaller meals, avoiding fatty foods, and staying hydrated can help. Your prescriber may also suggest taking semaglutide at a different time of day if nausea is problematic.
Can I prevent semaglutide side effects?
While you cannot prevent side effects entirely, certain strategies help: eat smaller, more frequent meals; avoid high-fat or spicy foods; stay hydrated; take the injection at a consistent time each week; and follow the gradual dose escalation schedule. Most side effects are mild and temporary. If severe, contact your GP immediately rather than stopping treatment suddenly.
What's the difference between semaglutide (Wegovy) and Ozempic side effects?
Both contain semaglutide but are different medications. Wegovy is licensed for weight loss; Ozempic is licensed for type 2 diabetes and contains a lower maximum dose (2 mg vs 2.4 mg). Side effects are similar, but the dose difference means Ozempic users typically experience fewer dose-related side effects. Never use Ozempic for weight loss — always use Wegovy if prescribed for weight management.
When should I contact my GP about semaglutide side effects?
Contact your GP if you experience severe stomach pain, signs of allergic reaction, changes in vision, or if side effects do not improve after 3–4 weeks. Also seek help if you cannot tolerate side effects or if they significantly affect your ability to eat or function. Your GP can assess whether semaglutide is still appropriate or if dose adjustment is needed.
Ready to take control of your weight loss journey?
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