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
What Happens on Mounjaro Day 4?
Day 4 of Mounjaro is typically when initial side effects become most noticeable for many people in the UK. This is the early phase of your treatment journey after your first 2.5 mg injection. Most people experience nausea, changes in appetite, or mild stomach discomfort during this period. These effects are usually temporary and often settle within the first week or two. Understanding what to expect can help you feel more confident about your tirzepatide treatment. Always contact your GP if you have concerns about how you're feeling.
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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 Early Side Effects on Day 4 of Mounjaro
By day 4 of your Mounjaro dose titration, your body is adjusting to the GIP and GLP-1 receptor agonist effects. Nausea is the most commonly reported side effect at this stage, affecting over 1 in 10 people. This typically peaks between days 2–5, then gradually improves. You may also notice reduced appetite — this is intentional and how the medication works, but it can feel unusual at first. Some people experience mild stomach discomfort or loose stools. These effects are not dangerous and do not mean the medication is harming you — they reflect how your gut is responding to the new hormone signalling.
- Nausea: usually mild to moderate, improves within 5–7 days for most people
- Reduced appetite: may feel strange but is the intended effect
- Mild stomach discomfort: often resolves without intervention
- Dizziness or fatigue: less common but can occur as your body adjusts
- Injection site reactions: minor redness or bruising at injection site
Managing these effects involves eating smaller, frequent meals and staying hydrated. Avoid fatty or spicy foods, which can worsen nausea. If symptoms are severe or worsening after day 5, contact your GP.
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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 — Red Flags on Day 4
Whilst mild nausea and reduced appetite are expected on day 4, certain symptoms warrant immediate GP contact. Severe, persistent vomiting (especially if you cannot keep fluids down) can lead to dehydration and requires medical assessment. Severe abdominal pain, particularly pain that spreads to your back, could indicate pancreatitis and needs urgent evaluation. Serious allergic reactions — swelling of the face, lips, throat, or difficulty breathing — require emergency care. Signs of diabetic complications like sudden vision changes also warrant urgent attention. Most people manage day 4 well with simple self-care, but your GP is always your first point of contact in the UK if anything concerns 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.
Is nausea on day 4 of Mounjaro normal?
Yes, nausea is very common in the first week of Mounjaro treatment, with clinical trials showing it affects over 1 in 10 people. It typically peaks around days 2–5 and improves significantly by day 7–10. Eating smaller meals, staying hydrated, and avoiding fatty foods can help. Contact your GP if it persists beyond a week or worsens.
Should I be worried if I feel dizzy on day 4?
Mild dizziness can occur as your body adjusts to tirzepatide, especially if you're eating less due to reduced appetite. Ensure you're staying hydrated and eating regular, smaller meals. If dizziness is severe, you feel faint, or it worsens, contact your GP to rule out other causes.
Can I take anti-nausea medication with Mounjaro?
Some anti-nausea medications can be used alongside Mounjaro, but this varies by individual. Always discuss options with your GP or the prescriber who issued your Mounjaro prescription. They can recommend the safest approach for your specific situation.
What should I eat on day 4 of Mounjaro?
Focus on bland, easily digested foods: plain rice, toast, soup, lean proteins, and fresh fruit. Avoid fatty, spicy, or rich foods that can trigger nausea. Eat smaller portions more frequently rather than large meals. Ginger tea and clear broths can help settle your stomach.
Is day 4 when Mounjaro starts working for weight loss?
Mounjaro begins affecting appetite and satiety from your first injection, so yes, day 4 is part of the active period. However, significant weight loss typically becomes apparent over weeks 2–4 as your body adjusts and appetite stabilises. Use our free GLP-1 weight loss calculator to explore realistic projections for your individual situation.
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.