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 side effects from Mounjaro (tirzepatide) are temporary and tend to improve within the first few weeks of treatment or after dose adjustments. Nausea, diarrhoea, constipation, and stomach pain are common initially but usually settle as your body adapts to the medication. However, the exact duration varies between individuals — some people experience mild effects that resolve quickly, whilst others may take longer to adjust. Understanding what to expect can help you manage the side effects more effectively and stay on track with your weight management journey using this prescription-only injection.
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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.
Common Side Effects and Their Timeline
The most frequently reported side effects of Mounjaro occur in the first 4–12 weeks of treatment:
- Nausea: Usually peaks during the first 2–4 weeks, then gradually improves. For most people, it resolves by week 8–12. Eating smaller, frequent meals and avoiding fatty or greasy foods can help reduce nausea.
- Diarrhoea and constipation: Often alternate during the adjustment phase. These typically settle within 4–8 weeks as your digestive system adapts. Staying hydrated and gradually increasing fibre intake (where diarrhoea is present) supports comfort.
- Stomach pain: Usually mild and temporary, improving within the first few weeks. Persistent or severe abdominal pain should be reported to your GP immediately as it may indicate pancreatitis.
- Reduced appetite: This often persists longer than other side effects — sometimes throughout treatment — but this is actually the intended effect of the medication.
Dose titration (gradually increasing your Mounjaro dose over weeks) is designed specifically to allow your body time to adjust, which typically reduces the severity and duration of side 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.
When to Contact Your GP and Long-Term Considerations
Most Mounjaro side effects are manageable and temporary, but contact your GP if nausea, vomiting, or stomach pain persist beyond 12 weeks, or if you experience severe symptoms earlier. Hair loss and fatigue can occasionally occur and may take longer to resolve. These are less common but should be discussed with your prescriber if they affect your quality of life.
Some side effects — particularly appetite suppression — are the desired pharmacological effect and will continue as long as you take the medication. Track your progress and any ongoing side effects using our free progress tracker to share detailed information with your GP or prescriber. Individual tolerance improves significantly by week 12–16 for most people, and many experience minimal side effects at their maintenance dose.
- 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 usually peaks in weeks 2–4 and improves by week 8–12 for most people. Eating smaller, frequent meals and avoiding fatty foods helps. If nausea persists beyond 12 weeks, contact your GP — dose adjustment may help.
Will I have side effects every time I increase my Mounjaro dose?
Many people experience temporary side effects when moving to a higher dose, but these are usually milder than the initial dose because your body has already begun adapting. Most settle within 1–2 weeks at the new dose level.
Are there side effects that last for the entire time I take Mounjaro?
Reduced appetite and occasional mild digestive changes may persist throughout treatment — this is the intended effect. However, severe or debilitating side effects should always be reported to your GP for assessment and possible dose adjustment.
What should I do if side effects don't improve after 12 weeks?
Contact your GP or your prescriber. Persistent side effects may respond to dose adjustment, dietary changes, or temporary dose pausing. Your prescriber can assess whether continuing treatment is appropriate for you.
Can I use the NHS BMI calculator to track my progress whilst managing Mounjaro side effects?
Yes — our free BMI calculator helps you monitor your weight and BMI over time, even during the adjustment phase when side effects may affect your appetite and weight loss rate. Track changes alongside any side effect patterns to share with your GP.
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.