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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Tirzepatide vs Wegovy Side Effects — What's the Difference?
Both tirzepatide (Mounjaro) and Wegovy (semaglutide) are prescription-only weekly weight loss injections licensed in the UK, but they work differently and have different side effect profiles. Tirzepatide is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 receptor agonist only. Understanding how each medication affects the body helps you and your prescriber make an informed choice during clinical assessment. This guide compares the side effects from clinical trial data and explains why individual experiences vary.
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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.
Clinical Trial Data: Side Effects Compared
Tirzepatide (Mounjaro) and Wegovy both cause similar common side effects, but the frequency and severity can differ because of their different mechanisms. In the SURMOUNT-1 trial, tirzepatide caused nausea in approximately 25–33% of participants at higher doses, whilst the STEP-1 trial showed Wegovy caused nausea in around 25% of participants. Both medications commonly cause gastrointestinal side effects: diarrhoea, constipation, vomiting, and stomach pain are reported by 20–40% of users. Injection site reactions (redness, bruising, mild pain) occur in 5–10% of people on either medication. Fatigue and dizziness are reported in 5–8% of users. Importantly, side effects are usually mild to moderate, tend to decrease over time as the body adjusts, and most people tolerate these medications well. Your prescriber can discuss specific risks during your clinical assessment.
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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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How Mechanism Differences Affect Side Effects
Tirzepatide's dual action on GIP and GLP-1 receptors may cause slightly different side effect patterns compared to Wegovy's single GLP-1 mechanism. Some clinical data suggests tirzepatide users report marginally higher nausea rates early in treatment, possibly because the dual mechanism has a stronger effect on appetite regulation and gastric emptying. However, both medications slow stomach emptying, which can cause bloating and indigestion in the first few weeks. The key point: individual responses vary widely — some people experience no nausea, whilst others find it manageable with dietary adjustments. Fatigue, headaches, and taste changes are reported with both, but are not assured. A prescriber will review your medical history and any previous medication side effects to help predict your likely experience.
- 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 tirzepatide safer than Wegovy in terms of side effects?
Both medications have similar safety profiles and side effect types. Neither is definitively "safer" — it depends on individual factors. Clinical trials show comparable rates of nausea, digestive issues, and injection site reactions. Your prescriber will assess which is more suitable based on your health history, other medications, and personal circumstances. Both require clinical assessment and ongoing monitoring.
Do side effects of tirzepatide get better over time?
Yes, most people find side effects decrease significantly after the first 4–8 weeks as the body adjusts. Nausea, the most common side effect, often improves as you progress through doses slowly. Staying hydrated, eating smaller meals, and avoiding fatty foods can help manage digestive symptoms early on. If side effects persist or worsen, inform your prescriber — dose adjustments or medication changes are possible.
Which causes more nausea — tirzepatide or Wegovy?
Clinical trials show similar nausea rates (around 25–33% on tirzepatide and 25% on Wegovy), but individual experience varies greatly. Some tirzepatide users report slightly higher nausea early in treatment, possibly due to the dual GIP+GLP-1 mechanism, but this isn't universal. Many people experience no nausea at all. Your GP or prescriber can discuss your specific risk factors during assessment.
Can I use our GLP-1 weight loss calculator to compare tirzepatide and Wegovy results?
Yes, our free GLP-1 weight loss calculator helps estimate potential weight loss at different doses for both medications. Remember that side effects don't necessarily correlate with effectiveness — some people with minimal side effects still achieve significant weight loss, whilst others manage more side effects for similar results. Clinical trial data shows tirzepatide averaged 15–clinically studied weight loss and Wegovy around 15%, but individual outcomes vary.
What serious side effects should I watch for with tirzepatide?
Serious side effects are rare but require immediate medical attention: severe stomach pain (possible pancreatitis), persistent vomiting, signs of allergic reaction (swelling of face or throat), changes in vision, or rapid heartbeat. These are uncommon but you should contact your GP or call 111 immediately if they occur. Always report any concerning symptoms to your prescriber — they are monitoring your safety throughout treatment.
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