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 2?
Day 2 of Mounjaro is typically when the initial injection begins to take effect in your body. Most people experience minimal symptoms on day 2, though some may notice early signs of nausea or mild digestive changes. The tirzepatide in your Mounjaro KwikPen starts working within hours, but full effects develop gradually over several days and weeks. This guide explains what to expect during day 2 and when you might need to contact your GP.
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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.
Early Effects and What to Expect on Day 2
On day 2 after your Mounjaro injection, the dual GIP and GLP-1 receptor agonist mechanism begins signalling to your brain and digestive system. Most people report no symptoms or only minor changes on day 2 — your body is adjusting to the new medication. Some users notice a subtle reduction in appetite or slight nausea, though this is not assured. The 2.5 mg starting dose is designed to be gentle on your system, allowing gradual adjustment. Common day 2 experiences include:
- No noticeable symptoms (most common)
- Mild nausea or queasiness
- Slight loss of appetite
- Minor stomach discomfort
- Slight fatigue or tiredness
- No change in energy or mood
Full side effects typically peak between days 3–7 as the medication reaches steady-state levels in your bloodstream. Individual responses vary widely — some people feel nothing for weeks, whilst others notice changes immediately.
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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 on Day 2
Most day 2 symptoms are normal and expected — nausea and mild stomach discomfort do not usually require urgent medical attention. However, contact your GP or NHS 111 immediately if you experience severe stomach pain, persistent vomiting that prevents you from keeping fluids down, signs of a serious allergic reaction (swelling of face, lips, or throat), or chest pain. If you have questions about whether your symptoms are normal, contact your prescriber — they can reassure you or adjust your care plan. Keep a simple symptom diary for the first few weeks to identify patterns and discuss with your healthcare team at follow-up appointments.
- 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 Mounjaro day 2 normal?
Nausea is possible but not assured on day 2. Many people experience no symptoms at all on day 2. If nausea does occur, it's usually mild and often improves over the first week as your body adjusts. Contact your GP if nausea is severe or prevents you from eating or drinking.
Can I take Mounjaro on an empty stomach on day 2?
Your injection itself is not affected by whether you've eaten. However, if you're experiencing nausea on day 2, eating small, light meals may help. Some people find that eating settles their stomach, whilst others prefer to eat less until nausea passes.
What should I do if I feel ill on Mounjaro day 2?
Minor side effects like mild nausea, tiredness, or slight stomach discomfort are common and usually pass within days. Rest, stay hydrated, and eat small meals if you can. Seek urgent help (call NHS 111) if you have severe pain, persistent vomiting, allergic symptoms, or chest pain.
How long does it take for Mounjaro to start working?
Tirzepatide begins working within hours of injection, but most people don't notice significant appetite suppression until days 3–7. Full effects develop over weeks and months. Weight loss typically becomes measurable after 2–4 weeks as the appetite-reducing effects strengthen.
Should I continue my normal diet on Mounjaro day 2?
Yes, continue eating normally unless you experience severe nausea. Mounjaro works alongside a reduced-calorie diet, not instead of one. If nausea prevents normal eating on day 2, eat small, easy-to-digest meals and resume your diet plan once symptoms settle. Use our GLP-1 weight loss calculator to plan your approach with Mounjaro.
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.