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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Mounjaro First Dose Side Effects vs Wegovy
Starting Mounjaro (tirzepatide) or Wegovy (semaglutide) often comes with side effects in the first weeks — nausea and digestive changes are most common. Both are GLP-1 based weight loss injections licensed in the UK, but they differ in mechanism and starting dose. This guide compares what to expect with each medication's first dose and helps you understand the difference between these prescription weight loss treatments. Use our free GLP-1 weight loss calculator to see projected outcomes at different doses, and speak with your GP or prescriber about which might suit you better.
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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.
How First Dose Side Effects Compare: Mounjaro vs Wegovy
Mounjaro (tirzepatide) starts at 2.5 mg once weekly — a dual GIP and GLP-1 receptor agonist. First dose side effects typically include mild nausea (affects many people), diarrhoea, constipation, and stomach discomfort. Most people experience these within the first few days and they often ease as the body adjusts within 1–2 weeks.
- Mounjaro mechanism: Dual receptor activation may cause more pronounced early appetite suppression
- Common first dose effects: Nausea, mild diarrhoea, loss of appetite, abdominal pain, fatigue
- Onset: Usually within 24–48 hours of injection
- Duration: Often improves by week 2–3 as your body adapts
Wegovy (semaglutide) starts at 0.25 mg once weekly — a single GLP-1 receptor agonist. Because the starting dose is much lower, first dose side effects are generally milder than Mounjaro. Nausea is still common but often less severe. Digestive symptoms like constipation or diarrhoea may occur but typically resolve quickly.
- Wegovy mechanism: Single GLP-1 activation with gradual dose escalation
- Common first dose effects: Mild nausea, headache, dizziness, constipation, reduced appetite
- Onset: Within 24–72 hours
- Duration: Usually settle within 1–2 weeks
Clinical trial data from SURMOUNT (Mounjaro) and STEP (Wegovy) trials both showed nausea and gastrointestinal effects as most frequent side effects, but individual tolerance varies considerably.
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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 Better?
The choice between Mounjaro and Wegovy isn't based on avoiding side effects — it's based on your individual health profile, eligibility, and how your body responds. Your GP or prescriber will assess your BMI, weight-related health conditions, and any contraindications. Use our free BMI calculator to understand your current weight status and eligibility context.
Neither medication is "better" — they work differently and suit different people. Mounjaro's dual mechanism may be more effective for some, whilst Wegovy's lower starting dose may suit those sensitive to GLP-1. Both require a UK prescription and clinical assessment. If you experience severe side effects with either medication, your prescriber can adjust your dose, pause treatment, or try a different approach. Managing side effects is part of the journey — most settle after the first 2–3 weeks as your body adapts.
- 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 with Mounjaro's first dose?
Nausea is the most frequent side effect reported in clinical trials. It typically appears within 24–48 hours of your first 2.5 mg injection and often improves significantly by week 2–3 as your body adjusts. Eating smaller, lighter meals and staying hydrated can help manage this.
Is Mounjaro's first dose worse than Wegovy's?
Not necessarily worse — just different. Mounjaro starts at a higher dose (2.5 mg vs Wegovy's 0.25 mg), so some people experience slightly more pronounced appetite suppression and nausea initially. However, individual experiences vary widely; some tolerate Mounjaro well whilst others find Wegovy easier to start with.
How long do first dose side effects usually last?
Most side effects settle within 1–2 weeks as your body adapts to the medication. Nausea, diarrhoea, and constipation are temporary in the majority of cases. If side effects persist beyond week 3 or are severe, contact your prescriber — dose adjustments or alternative approaches may help.
Can I reduce nausea from Mounjaro's first dose?
Yes. Eat small, frequent, bland meals; stay well hydrated; avoid fatty or spicy foods; and take your time eating. Some people find ginger tea or anti-nausea remedies helpful. If nausea is severe, contact your prescriber — they can offer advice or consider pausing the next dose.
Which medication has fewer side effects overall?
Both medications have similar side effect profiles — nausea and digestive changes are common with both. Wegovy's lower starting dose may mean a gentler introduction for some people, whilst Mounjaro's dual mechanism suits others better. Your prescriber will recommend based on your individual health, not side effect avoidance alone.
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.