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?
Most people taking Mounjaro (tirzepatide) experience some side effects, but they are usually temporary and improve within 4 to 8 weeks as your body adjusts to the medication. The most common side effects — nausea, diarrhoea, vomiting, and constipation — tend to be mild to moderate at the start and reduce significantly over time. However, side effect duration varies between individuals, and some people experience fewer effects than others. Understanding what to expect can help you manage them effectively while your body acclimatises to this prescription-only weight management injection.
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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.
Timeline: When Mounjaro Side Effects Typically Improve
Side effects on Mounjaro follow a predictable pattern for most users. In the first 1–2 weeks after starting or increasing your dose, side effects are most likely to occur and be most noticeable. Nausea and stomach upset are especially common during this period.
Weeks 2–4: Many people notice improvement as their body begins to adjust. Nausea may reduce to mild queasiness, and appetite suppression becomes more manageable.
Weeks 4–8: Most side effects have significantly improved or resolved completely for the majority of users. You should be feeling much better by week 8 on each dose.
After 8 weeks: If you remain on the same dose without further escalation, side effects typically continue to improve. However, if your dose is increased (as part of the standard dose titration protocol), side effects may briefly return at a lower intensity before improving again.
For those taking Mounjaro via KwikPen, keeping a simple log of how you feel each week can help you notice improvements and predict when symptoms will ease.
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.
Managing Side Effects While They Last
While waiting for side effects to settle, practical strategies can help. For nausea: eat smaller, frequent meals; avoid fatty or rich foods; stay hydrated with water and herbal tea. For diarrhoea: increase fibre and fluids gradually, and avoid sudden dietary changes. For constipation: increase water intake and gentle movement like walking.
If side effects are severe or don't improve by week 8, contact your prescriber — they may suggest adjusting your dose, staying on your current dose longer before escalating, or other management strategies. Always discuss any concerns with your GP or the healthcare professional who prescribed your Mounjaro, as they can provide personalised guidance based on your individual health.
- 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.
Do Mounjaro side effects get worse if you increase your dose?
Yes, side effects often briefly return or intensify when you increase your dose, as part of the dose titration process. However, they typically improve again within 4–8 weeks on the new dose. This is normal and expected. Your prescriber will space dose increases by at least 4 weeks to allow adjustment time.
How long does nausea last on Mounjaro specifically?
Nausea is usually most noticeable in weeks 1–3 and improves significantly by week 4–6 for most people. However, some experience mild nausea for 8+ weeks, whilst others have minimal nausea from the start. Individual variation is significant.
Can you stop Mounjaro if side effects don't improve?
Yes, you can discuss stopping Mounjaro with your prescriber if side effects are intolerable or don't improve. Your prescriber may also suggest staying on your current dose longer before escalating, or switching to a different weight management medication like Wegovy (semaglutide).
Are Mounjaro side effects permanent?
No, Mounjaro side effects are temporary and reversible. They improve within days to weeks of your body adjusting to the medication, or after stopping it. There are no permanent side effects from Mounjaro itself, though some effects (like stomach upset) may persist if you have underlying digestive conditions.
Does everyone experience side effects on Mounjaro?
No. Some people experience minimal or no side effects, whilst others experience common effects like nausea or constipation. Side effect experience varies based on individual metabolism, dose, eating habits, and overall health. Your GP or prescriber can discuss what to expect in your specific case.
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.