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 — How Wegovy Compares to Mounjaro
Semaglutide is a GLP-1 receptor agonist prescribed as Wegovy for weight management in UK adults. Both semaglutide and tirzepatide (Mounjaro) are prescription-only weekly injections used for weight loss, but they work differently and have different side effect profiles. Understanding how these medications compare can help you and your GP discuss which might be suitable for you. This guide covers the clinical evidence, side effects of both, and what you need to know before considering either treatment.
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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.
Clinical Trial Data and Side Effect Comparison
Semaglutide (Wegovy) was evaluated in the STEP trials, where participants taking the 2.4mg dose experienced an average weight loss of approximately 14.9% of body weight over 68 weeks, alongside a reduced-calorie diet and physical activity. Mounjaro (tirzepatide) was tested in the SURMOUNT-1 trial, showing average weight loss of 20.9% at the maximum 15mg dose — though this represents a dual GIP and GLP-1 mechanism versus semaglutide's single GLP-1 pathway.
Side effect incidence is similar between both medications:
- Nausea and vomiting are the most common — more frequent in early weeks and often reduce over time
- Diarrhoea and constipation both reported, though patterns vary individually
- Stomach pain and indigestion occur in a significant proportion of users
- Injection site reactions (redness, bruising) are usually mild and temporary
- Hair loss (alopecia) reported by some users, typically reversible after stopping
Both medications carry small risks of serious side effects including pancreatitis and gallbladder complications — seek urgent medical attention if you develop severe, persistent stomach pain.
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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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Which Medication Might Suit You — A Prescriber Decision
Choosing between semaglutide and Mounjaro is not a decision you make alone. A licensed prescriber will assess your individual health, medical history, previous weight loss attempts, and any existing health conditions to determine which is more appropriate. Both require a prescription following a clinical assessment — neither is available over the counter or without professional review.
Try our free GLP-1 weight loss calculator above to explore potential weight loss projections at different doses of either medication. Use our BMI calculator to check your current status and discuss the results with your GP or a licensed prescriber. Individual responses to both medications vary considerably — some people lose more weight on one versus the other, and side effect tolerance differs significantly between individuals.
- 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.
Is semaglutide safer than tirzepatide (Mounjaro)?
Both are prescription-only, licensed medications with similar safety profiles when used as directed. Clinical trials show comparable side effect rates. Safety depends on individual health factors and prescriber assessment. Neither is inherently "safer" — suitability is individual and determined by your GP or prescriber based on your health history.
Why do people report fewer side effects with one versus the other?
Individual response to medications varies significantly. Some people tolerate GLP-1 mechanisms better than dual GIP+GLP-1, and vice versa. Starting doses, dose escalation speed, and concurrent lifestyle factors all influence side effect experience. Your prescriber may adjust dosing or switch between medications based on your tolerance.
How long do semaglutide (Wegovy) side effects last?
Most common side effects like nausea typically peak in the first 2–4 weeks and improve as your body adjusts. Some people experience ongoing nausea at higher doses. Serious side effects require immediate medical attention. If side effects persist or worsen after 4 weeks, contact your prescriber — dose adjustment may help.
Can you switch from semaglutide to Mounjaro if side effects are problematic?
Yes, switching between medications is possible and managed by your prescriber. There is no official established conversion — your prescriber will determine an appropriate starting dose with the new medication. Any switch requires clinical reassessment and guidance from your GP or licensed prescriber.
Are weight loss injections safe for long-term use?
Both semaglutide and tirzepatide are designed for ongoing use, with clinical trials extending beyond a year. Long-term safety data continues to accumulate. Your prescriber will monitor you regularly and discuss ongoing use at routine reviews. The decision to continue is based on your response, side effect tolerance, and health outcomes.
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.