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
How Long Do Mounjaro Side Effects Last?
Mounjaro side effects typically improve within a few weeks as your body adjusts to the tirzepatide dose, though timing varies between individuals. Most common effects like nausea and diarrhoea tend to ease after 4–8 weeks on each dose, especially during the dose titration phase. However, some effects can persist longer, and a small number of people experience ongoing symptoms. Understanding what to expect helps you decide whether this prescription-only medication is right for you alongside lifestyle changes.
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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.
Mounjaro vs Wegovy: Side Effect Duration and Trial Data
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both once-weekly GLP-1 receptor agonist injections licensed in the UK for weight management, but they work differently. Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy activates only the GLP-1 pathway. In clinical trials, both medications showed similar side effect patterns: nausea, diarrhoea, and vomiting were most common in the first 4–8 weeks, then typically improved. The SURMOUNT trial for Mounjaro and STEP trials for Wegovy reported that gastrointestinal side effects were more frequent at higher doses and during the initial dose titration phase. Most participants who continued treatment found side effects manageable, though individual experiences vary significantly. Your prescriber will monitor you closely during the dose escalation schedule to identify any persistent effects.
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Timeline: What to Expect Week by Week
Weeks 1–4 (starting dose): Many people experience mild nausea or appetite changes; this is often the most noticeable period. Weeks 5–8 (dose increase): Side effects may temporarily worsen when the dose increases, then improve again over 2–4 weeks. Weeks 9–16: Effects typically become less severe as your body adjusts; many people report nausea reducing significantly. Beyond week 16: For those who reach maintenance dose, most side effects have either resolved or become mild and manageable. If effects persist beyond 8 weeks on the same dose, speak to your GP or prescriber—dose adjustment or a switch to Wegovy may be considered. Using our GLP-1 weight loss calculator can help you understand what to expect at different doses.
- 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 does Mounjaro nausea typically last?
Nausea is the most common side effect and usually peaks in the first 1–2 weeks after injection, then gradually improves over 3–6 weeks. For many people, it eases significantly by week 4–8. If nausea persists beyond 8 weeks on the same dose, contact your GP—dosing adjustments or anti-nausea medication may help.
Will Mounjaro side effects go away completely?
For most people, yes. Clinical trials showed that gastrointestinal side effects improve substantially after the first 4–8 weeks on each dose. However, a small number of people experience ongoing mild symptoms even at maintenance dose. Your prescriber will help you decide if continuing is right for you.
Do Mounjaro side effects last longer than Wegovy side effects?
Clinical trial data suggests similar timelines: both medications show peak side effects in weeks 1–4 and gradual improvement thereafter. However, individual variation is significant. Some people find Mounjaro effects settle faster; others prefer Wegovy. Your prescriber will recommend the best option based on your health profile.
What if my Mounjaro side effects don't improve after 8 weeks?
If side effects persist or worsen beyond 8 weeks on the same dose, speak to your prescriber. Options include pausing dose escalation, reducing the dose, switching to Wegovy, or stopping treatment. Never adjust your Mounjaro KwikPen dose without medical guidance.
Can I speed up the Mounjaro dose titration to reduce side effects faster?
No—the dose escalation schedule (minimum 4 weeks per dose) is designed to help your body adjust gradually and minimise side effects. Accelerating the schedule increases the risk of severe nausea and diarrhoea. Stick to the recommended timeline and discuss any concerns with your GP or prescriber.
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.