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 on Day 3 of Mounjaro
If you've just started your Mounjaro treatment or are about to begin, you may be wondering what day 3 will feel like. Most people taking this tirzepatide injection start with the 2.5 mg dose and experience their first potential side effects within the first few days. This guide explains what's common on day 3, how the NHS supports Mounjaro users in the UK, and whether what you're experiencing is normal. Remember that individual experiences vary significantly, and your prescriber or GP can give you personalised advice about your specific situation.
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.
NHS Mounjaro Support and What Day 3 Typically Involves
By day 3 of your Mounjaro (tirzepatide) treatment, your body is beginning to process the 2.5 mg starting dose. The most commonly reported experience on day 3 is mild to moderate nausea, which peaks in many people around 48–72 hours after injection. This is not dangerous but can be uncomfortable.
- Nausea — the most frequent early side effect; usually mild on day 3
- Reduced appetite — some people notice this immediately
- Mild stomach discomfort or indigestion
- Possible mild diarrhoea or constipation
- Fatigue or tiredness — particularly if nausea has affected sleep
These experiences are not signs that the medication isn't working — they're normal early responses as your system adjusts. If you're accessing Mounjaro through NHS tier 3 specialist weight management services, your clinical team will have provided guidance on managing early side effects. Private prescribers also provide detailed information on managing the first week. Most people find that nausea and other initial side effects settle within 5–7 days as the body adapts.
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 Day 3 and When to Contact Your NHS GP or Prescriber
On day 3, focus on staying hydrated and eating small, bland meals if nausea is present. Ginger tea, plain crackers, and avoiding fatty or spicy foods often help. Most nausea is manageable without medication, though your GP can prescribe anti-nausea medication if symptoms are severe.
Contact your GP or prescriber immediately if you experience severe stomach pain spreading to your back (possible pancreatitis), persistent vomiting preventing fluid intake, severe allergic reaction (facial swelling), or chest pain. These are rare but require urgent medical attention. For routine side effects like mild nausea or loss of appetite, note them and discuss at your next follow-up appointment. Use our progress tracker to log how you're feeling each day — this helps you and your prescriber spot patterns and adjust your management plan if needed.
- 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 3 of Mounjaro normal?
Yes, nausea is very common on day 3. Clinical trials show that gastrointestinal side effects like nausea peak around 48–72 hours after the injection and typically settle within 5–7 days as your body adjusts. If nausea is severe or persistent beyond day 7, contact your prescriber or GP.
Should I continue with Mounjaro if I feel unwell on day 3?
Mild side effects like nausea and reduced appetite are normal and do not mean you should stop. However, if you have concerns, contact your prescriber before your next dose. Severe symptoms like vomiting, inability to keep fluids down, or severe stomach pain require urgent medical advice.
Can I access NHS Mounjaro support if I'm struggling on day 3?
If you received Mounjaro through NHS tier 3 services, your specialist team provides support during the initial weeks. Contact them directly with concerns. If you have a private prescription, discuss side effects with your prescriber. Your GP can also provide general support even if they didn't prescribe it.
What's the difference between Mounjaro and Wegovy side effects in early days?
Both are GLP-1 based medications, but Mounjaro is a dual GIP and GLP-1 agonist while Wegovy is GLP-1 only. Mounjaro users may experience slightly different nausea patterns, but early side effects are broadly similar. Individual responses vary, so comparison isn't always helpful — focus on your own experience.
How do I know if day 3 symptoms are serious?
Most day 3 symptoms (mild nausea, reduced appetite, fatigue) are not serious. Seek urgent medical attention only for severe stomach pain, inability to keep fluids down, signs of allergic reaction, or chest pain. For routine discomfort, contact your prescriber or GP within 24 hours rather than waiting.
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.