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
Free tracker included with every download this week
Mounjaro 2.5 mg vs Wegovy: Side Effects Compared
If you're considering tirzepatide or semaglutide for weight management in the UK, understanding the side effect profile is essential. Mounjaro 2.5 mg (the starting dose of tirzepatide) and Wegovy have different mechanisms and tolerability patterns. This guide compares real side effect data from clinical trials, helps you understand what to expect, and explains why a prescriber will assess suitability individually. Both are prescription-only weight loss injections requiring clinical assessment before use.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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: SURMOUNT vs STEP Side Effects
Mounjaro (tirzepatide) clinical trial data comes from the SURMOUNT-1 study, which tracked side effects at the 2.5 mg starting dose and higher maintenance doses. Wegovy (semaglutide) data comes from the STEP trials using the same methodology.
- Nausea: Both very common at initiation. SURMOUNT-1 reported nausea in over 25% of participants at 2.5 mg; STEP trials showed similar rates with semaglutide 0.25–0.5 mg starting doses.
- Gastrointestinal effects: Diarrhoea, constipation, and vomiting occurred in both groups. Mounjaro participants reported slightly higher rates of stomach pain (abdominal discomfort) in early weeks.
- Mechanism difference: Mounjaro's dual GIP + GLP-1 action may cause different gastric effects compared to Wegovy's GLP-1-only mechanism, though both slow stomach emptying.
- Dose escalation matters: Most side effects peak at initiation and dose increases, then stabilise. The 2.5 mg starting dose is designed to minimise side effects before escalating.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Individual Tolerability and Prescriber Assessment
Not everyone experiences the same side effects — some people tolerate Mounjaro well from the start, while others find Wegovy's gentler dose escalation easier to manage. Your prescriber will assess your medical history, current medications, and any gut sensitivity when deciding which is suitable. If you develop side effects on one medication, switching to the other may be considered, but this requires medical guidance. Using our free GLP-1 weight loss calculator can help you understand projected outcomes at different doses alongside tolerability expectations. Both medications are available through NHS specialist services (with regional variation) or private prescribing with clinical assessment.
- 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.
What's the most common side effect of Mounjaro 2.5 mg?
Nausea is the most common side effect, reported in over 25% of trial participants at the 2.5 mg starting dose. It typically peaks within the first few days of injection and often improves as your body adjusts. Eating smaller meals and staying hydrated can help manage nausea.
Is Mounjaro 2.5 mg harder to tolerate than Wegovy's starting dose?
Not necessarily. Both have similar nausea rates at initiation. Mounjaro uses a slower titration schedule (4 weeks per dose), while Wegovy escalates weekly. Some people find Mounjaro's slower pace easier; others prefer Wegovy's faster titration. Individual tolerance varies.
Can I switch from Mounjaro to Wegovy if side effects are too much?
Yes, switching is possible and sometimes done in clinical practice. However, this requires your prescriber's approval and should be managed carefully to avoid withdrawal effects. Never stop or switch medications without medical guidance.
When do Mounjaro 2.5 mg side effects usually stop?
Most gastrointestinal side effects peak in the first 1–2 weeks and improve within 4 weeks as your body adapts. Some people experience ongoing mild nausea, which often resolves when doses are increased. Your prescriber will monitor your response.
Does Mounjaro 2.5 mg cause weight loss faster than Wegovy with fewer side effects?
Not necessarily. Both medications support similar weight loss averages when used correctly alongside diet and exercise. Side effect experience is highly individual and doesn't predict weight loss success. Clinical trials showed comparable tolerability and outcomes between the two medications.
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.