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 Day 1 of Mounjaro Treatment?
Many people starting Mounjaro (tirzepatide) experience little to no side effects on day 1, though some may notice mild symptoms within the first few hours or over the following days. The starting dose of 2.5 mg is deliberately low to allow your body to adjust gradually. While serious side effects are rare, nausea, dizziness, and injection site reactions can occur. Understanding what to expect helps you prepare and know when to contact your GP. This guide covers NHS-aligned information about Mounjaro's initial effects.
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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 Day 1 and Early-Week Side Effects
The first dose of Mounjaro (2.5 mg) is an adjustment dose — designed to be tolerated well by most people. Many users report feeling fine on day 1, while others experience mild effects that develop gradually over 24–72 hours.
Most commonly reported early effects:
- Mild nausea (often worse in the morning or after eating fatty foods)
- Slight dizziness or lightheadedness, especially when standing quickly
- Loss of appetite or reduced interest in food
- Mild stomach discomfort or indigestion
- Injection site reactions (redness, slight bruising, minor pain at injection point)
- Fatigue or tiredness
These effects are usually mild at the 2.5 mg starting dose and often settle within a few days as your body adjusts to the medication. Individual responses vary significantly — some people feel no different at all.
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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
Most day 1 side effects resolve without treatment, but contact your GP or NHS 111 immediately if you experience severe or persistent symptoms:
- Severe stomach pain spreading to your back (possible pancreatitis)
- Serious allergic reaction (swelling of face, lips, or throat, difficulty breathing)
- Severe vomiting lasting more than a few hours
- Chest pain or significant changes in heart rate
- Signs of dehydration (severe dizziness, confusion, very dark urine)
These serious effects are rare but require urgent medical attention. Your NHS prescriber can adjust your dose or medication plan if side effects are troublesome. Use our free GLP-1 weight loss calculator to track your progress and discuss results with your GP.
- 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 on day 1 of Mounjaro?
No — many people experience no side effects on day 1 at all. The 2.5 mg starting dose is deliberately low. Some people feel fine for days before any mild effects appear, whilst others never notice any symptoms. Individual responses vary significantly based on genetics, metabolism, and stomach sensitivity.
Is nausea on day 1 of Mounjaro normal?
Yes, nausea is one of the most commonly reported early effects of Mounjaro, though it doesn't happen to everyone. If it occurs, it's usually mild and often improves within a few days as your body adjusts. Eating smaller, less fatty meals and staying hydrated can help. Contact your GP if severe nausea persists beyond a week.
What's the difference between day 1 effects and allergic reactions?
Day 1 side effects like mild nausea or dizziness are expected and temporary. True allergic reactions are rare but serious — they involve swelling of the face, lips, or throat, difficulty breathing, or a widespread rash. If you experience these symptoms, seek immediate medical attention via NHS 111 or A&E. Never delay if you suspect an allergic reaction.
Can I take painkillers or anti-sickness tablets with my first Mounjaro dose?
You should discuss this with your prescriber or pharmacist before day 1. Some medications may interact with Mounjaro or affect how well it works. Anti-sickness tablets can be helpful for mild nausea, but your prescriber needs to know what you're taking. Always check with your GP or pharmacist before adding any new medication.
How long does it take for Mounjaro side effects to settle after day 1?
Most day 1 or early-week side effects settle within 3–7 days as your body adjusts. However, individual timelines vary — some people feel better after 24 hours, whilst others take 1–2 weeks. If side effects persist beyond two weeks or worsen, contact your GP. They can advise whether a dose adjustment is needed or discuss alternative options with you.
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.