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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Wegovy Weight Loss Side Effects — What You Need to Know
Wegovy (semaglutide) is a prescription-only weight loss injection licensed in the UK for adults with a BMI of 30 or above, or 27 or above with weight-related health conditions. Like all medications, Wegovy can cause side effects. Most side effects are mild and improve over time as your body adjusts, but it's important to understand what to expect. This guide covers the common and serious side effects reported in clinical trials, how long they typically last, and when to 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 Side Effects of Wegovy
In the STEP-1 clinical trial, the most frequently reported side effects of semaglutide were gastrointestinal (gut-related). These include nausea, diarrhoea, vomiting, and constipation — with nausea being the most common. Most people experience these side effects during the dose escalation phase (the first 4–8 weeks) and find they improve significantly once their body adjusts.
- Nausea: Very common, often worst in the first 1–2 weeks of each dose increase. Usually improves within 2–4 weeks
- Diarrhoea and constipation: Can alternate. Staying hydrated and eating small, regular meals helps
- Vomiting: Less common than nausea but can occur, especially if you eat large meals
- Stomach pain: Mild abdominal discomfort is common. Severe pain requires urgent medical attention
- Reduced appetite: This is actually the intended effect, but some people find it uncomfortable at first
- Injection site reactions: Redness, bruising, or mild pain where you inject. Usually mild and temporary
These side effects typically decrease over 4–8 weeks as your body adapts 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.
Free Weight Loss Tools & Guides
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Less Common and Serious Side Effects
While uncommon, some more serious side effects have been reported and require immediate medical attention. Severe, persistent stomach pain spreading to your back may indicate pancreatitis (inflammation of the pancreas) — seek emergency care immediately. Gallbladder problems (pain in the upper right abdomen) and serious allergic reactions (swelling of face, lips, tongue, or throat) are rare but serious.
Other reported side effects include headaches, dizziness, fatigue, acid reflux, and hair loss (alopecia). If you have changes in your vision, especially if you have diabetes, contact your GP urgently. Most people tolerate Wegovy well, but individual responses vary. Always report any concerning symptoms to your prescriber.
- 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 Wegovy side effects last?
Most side effects are worst during the first 1–2 weeks after starting or increasing your dose, and typically improve significantly within 2–4 weeks as your body adjusts. Nausea is usually the longest-lasting side effect but often becomes manageable. If side effects persist beyond 4 weeks or worsen, contact your GP.
Can I reduce Wegovy side effects?
Yes. Eat small, frequent meals rather than large ones. Stay well hydrated. Avoid fatty or greasy foods, which can trigger nausea. Slow dose increases (your prescriber may increase more gradually) can help. If side effects are severe, your prescriber may recommend pausing at your current dose before increasing further.
Is Wegovy the same as Ozempic?
Both contain semaglutide, but Wegovy is specifically licensed in the UK for weight management (doses up to 2.4 mg), while Ozempic is licensed for type 2 diabetes (doses up to 2 mg). Ozempic is not licensed for weight loss in the UK. Always ensure your prescription is for Wegovy, not Ozempic.
When should I contact my GP about Wegovy side effects?
Contact your GP if you experience severe, persistent stomach pain (especially spreading to your back), serious allergic reactions, gallbladder pain, vision changes, or any side effect that doesn't improve after 4 weeks or is severely affecting your daily life. Seek emergency care for severe symptoms.
Can I stop Wegovy if side effects are too much?
Never stop your injection suddenly without speaking to your prescriber first. If side effects are intolerable, your prescriber may recommend pausing treatment, reducing to a lower dose, or stopping gradually. This must be managed by your healthcare team to ensure your safety.
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.