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 Are Mounjaro Day 2 Side Effects?
Many people starting Mounjaro (tirzepatide) wonder what side effects to expect in the first few days after their initial injection. Day 2 is often when early reactions become noticeable. The most common side effects at this stage are nausea, mild stomach discomfort, and fatigue. These typically reflect how your body is beginning to respond to the dual GIP and GLP-1 receptor agonist mechanism. Most day 2 side effects are mild and temporary, but understanding what's normal helps you manage your treatment confidently and know when to contact your GP.
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 Day 2 Side Effects and What They Mean
On day 2 of your Mounjaro KwikPen injection, you may experience several side effects as your body adapts to the medication:
- Nausea — The most frequently reported early side effect. Usually mild on day 2 and often improves as your body adjusts over the first 2–4 weeks.
- Stomach pain or discomfort — Mild abdominal discomfort is common. This relates to how tirzepatide slows gastric emptying (the rate food leaves your stomach).
- Fatigue or tiredness — Some people feel unusually tired on day 2. This typically resolves within 24–48 hours.
- Loss of appetite — By design, Mounjaro reduces hunger signals. On day 2, you may notice you're less interested in food than usual.
- Mild headache or dizziness — Less common on day 2 specifically, but can occur as your body responds to the new medication.
These side effects reflect the medication working as intended. However, severity varies significantly between individuals — some experience minimal symptoms while others notice more pronounced effects. Track your symptoms using our free progress tracker to identify patterns and share them with your GP.
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.
When to Contact Your GP About Day 2 Symptoms
Most day 2 side effects are expected and manageable, but certain symptoms require prompt medical attention. Contact your GP immediately if you experience severe, persistent stomach pain (especially pain spreading to your back, which may indicate pancreatitis), severe vomiting that prevents you from keeping fluids down, signs of a serious allergic reaction (swelling of face, lips, or throat), or rapid or irregular heartbeat.
For moderate symptoms like nausea or stomach discomfort, contact your GP if they don't improve within 3–4 days, or if they worsen. Your prescriber may adjust your approach — such as taking smaller meals, changing meal timing, or adjusting hydration. Remember: you're on the starting dose (2.5 mg) of tirzepatide, which is the lowest available. This dose is designed to allow your body time to adapt before any dose escalation.
- 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 2 of Mounjaro normal?
Yes, nausea is very common on day 2 and in the first few weeks of Mounjaro treatment. It's one of the most frequently reported early side effects and typically improves as your body adjusts. If nausea persists beyond a week or is severe, contact your GP for support strategies.
Should I take my second Mounjaro dose if I had side effects on day 2?
Yes, unless your GP advises otherwise. Day 2 side effects do not mean you should skip your next dose. Continue your weekly injections as prescribed. Side effects typically reduce significantly by week 3–4 as your body adapts. Always discuss concerns with your prescriber.
How does the dose titration process help with side effects?
Mounjaro uses a dose titration schedule starting at 2.5 mg, increasing every 4 weeks to allow your body to adjust gradually. This approach helps minimise early side effects. You remain on each dose for at least 4 weeks before escalating, giving your system time to adapt before stronger effects occur.
Is day 2 nausea different from nausea later in treatment?
Day 2 nausea is typically mild and related to initial medication exposure. Later nausea (weeks 2–4+) may relate to appetite suppression and slower gastric emptying. Clinical trials show most people adapt within 4 weeks. If nausea persists beyond this, speak to your GP about management strategies.
Can I manage day 2 side effects with diet or lifestyle changes?
Yes — eat smaller, frequent meals rather than large portions; stay hydrated; avoid greasy or rich foods; and rest if you feel fatigued. Ginger tea or peppermint may help with mild nausea. However, always consult your GP before taking additional supplements or medications alongside 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.