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 to Expect in Week 2 On Mounjaro
During week 2 on Mounjaro, you're still on the starting dose (2.5 mg) and your body is adjusting to tirzepatide, the active ingredient. Most people experience initial side effects like nausea, stomach discomfort, or changes in appetite during this early phase — these are common and often improve as your body adapts. It's important to understand what's normal, when to contact your GP, and how to manage symptoms. This guide covers what to expect during your second week on this prescription-only weight management injection.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 Experiences in Week 2
Week 2 is typically when initial side effects are most noticeable. You may experience nausea (the most commonly reported side effect), changes in appetite, mild stomach pain, or changes to bowel habits — constipation or diarrhoea can both occur. Some people feel unusual tiredness or dizziness. These side effects happen because tirzepatide affects how your stomach empties and signals fullness to your brain.
Most of these effects are temporary and tend to reduce by weeks 3–4 as your body adjusts. However, severity varies significantly between individuals — some people notice minimal side effects whilst others experience more pronounced symptoms. It's crucial to stay hydrated, eat small frequent meals, and avoid fatty or spicy foods, which can worsen nausea. If you're struggling with side effects, contact your GP or the prescriber who issued your Mounjaro — they may suggest dietary changes or over-the-counter remedies. Never stop taking your injection without medical guidance.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Managing Your First Few Weeks
During week 2, focus on tolerability rather than weight loss — you're still establishing how your body responds to tirzepatide. Keep a simple food diary to identify which foods trigger nausea. Ginger tea, small frequent meals, and staying upright after eating can help manage early side effects. Ensure you're drinking plenty of water, as dehydration worsens nausea and dizziness.
Your next injection is due at the end of week 4. You'll stay on the 2.5 mg starting dose for the full 4 weeks before any dose increase. If side effects are severe or you have concerns — such as persistent vomiting, severe stomach pain, or signs of allergic reaction (swelling of face, lips, or throat) — contact your GP immediately. Track how you feel each day using our free progress tracker to help you remember details when speaking with your healthcare provider.
- 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 normal in week 2 of Mounjaro?
Yes, nausea is the most commonly reported side effect during the first few weeks on Mounjaro. It typically improves by weeks 3–4 as your body adjusts to tirzepatide. Eating smaller, frequent meals and avoiding fatty foods can help. If nausea is severe or persists, contact your GP.
Should I expect weight loss by week 2?
Weight loss is usually minimal in week 2 — most people are still in the adjustment phase. Significant weight loss typically begins in weeks 4–8. Clinical trials showed meaningful weight loss from week 4 onwards. Focus on tolerating the medication first, rather than expecting immediate results.
Can I change my diet in week 2 on Mounjaro?
Yes, you can and should adjust your diet. Eat smaller, more frequent meals, stay hydrated, and avoid fatty or spicy foods that may trigger nausea. Many people naturally eat less because tirzepatide reduces appetite and increases feelings of fullness — listen to your body's hunger signals.
What side effects mean I should contact my GP immediately?
Contact your GP urgently if you experience severe stomach pain (possibly indicating pancreatitis), persistent vomiting that prevents you keeping food down, signs of allergic reaction (swelling of face, lips, or throat), or any other symptom that concerns you. Mild nausea and appetite changes are normal, but severe symptoms need medical evaluation.
Will my side effects improve after week 2?
For most people, yes — side effects typically peak in weeks 1–2 and gradually improve by week 4. However, this varies between individuals. Some people experience persistent mild nausea throughout their treatment, whilst others notice rapid improvement. Your body will continue adjusting as you progress through dose titration.
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.