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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When Do Mounjaro Side Effects Start?
Side effects from Mounjaro (tirzepatide) can start within the first few days of your initial injection, though the timing and severity vary between individuals. Most people experience nausea or stomach discomfort during the first week, particularly after increasing your dose. Understanding the typical timeline helps you prepare and know when to seek GP advice. This guide explains when Mounjaro side effects usually begin, how long they typically last, and practical steps you can take to manage them during your treatment.
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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.
When Side Effects Typically Begin
Most side effects from Mounjaro appear within 24 to 72 hours of your injection, though some people notice changes within hours. Nausea is the most commonly reported early side effect—it often develops on the day of injection or the following day. Stomach pain, diarrhoea, and constipation typically emerge within the first week of starting or increasing your dose.
- Days 1–3: Nausea, mild stomach discomfort, and appetite changes most common
- Days 3–7: Gastrointestinal effects (constipation, diarrhoea, indigestion) peak in many users
- Week 2 onwards: Most side effects begin to improve as your body adjusts
It's important to note that side effects are usually temporary and dose-dependent. Your prescriber will help you titrate Mounjaro gradually—starting at 2.5 mg and increasing every four weeks—to allow your body time to adapt.
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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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Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Managing Side Effects and When to Seek Help
Most Mounjaro side effects settle within 7 to 10 days as your body adjusts to the medication. Simple steps like eating smaller, lighter meals, staying hydrated, and avoiding fatty or greasy foods can significantly reduce nausea and stomach pain. If side effects persist beyond two weeks or are severe, contact your prescriber immediately.
Seek urgent GP advice if you experience: severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions, gallbladder symptoms, or changes in vision. These require medical evaluation. Using our free GLP-1 weight loss calculator can help you understand realistic timelines for results—remember that side effects are a normal part of dose adjustment, not a sign the medication isn't working for you.
- 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.
Will I definitely get side effects from Mounjaro?
No—not everyone experiences side effects, and severity varies widely. Some users report minimal nausea or stomach discomfort, while others experience more noticeable effects. Starting at a low dose (2.5 mg) and increasing gradually helps your body adjust and often reduces side effect intensity.
How long do Mounjaro side effects usually last?
Most side effects peak during the first 3–7 days after injection, then improve significantly by day 10–14 as your body adapts. If you're increasing your dose, you may experience a brief return of mild symptoms, which again typically settle within a week.
Can I take anything to manage Mounjaro nausea?
Speak to your GP or prescriber before taking any anti-nausea medication. Simple strategies like eating smaller meals, avoiding fatty foods, staying hydrated, and resting often help. Ginger supplements may also be worth discussing with your prescriber as a potential aid.
Is Mounjaro the same as Wegovy when it comes to side effects?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are different medications with similar side effect profiles—nausea, stomach discomfort, and diarrhoea are common with both. However, Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy uses GLP-1 alone, so individual experiences differ. Your prescriber can help determine which is more suitable for you.
What should I do if side effects don't improve after two weeks?
Contact your prescriber or GP. Persistent side effects may indicate you need to stay at your current dose longer before increasing, or your prescriber may recommend pausing treatment temporarily. Mounjaro is a UK prescription-only medication, and your prescriber can adjust your plan based on how you're responding.
Ready to take control of your weight loss journey?
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