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
When Do Mounjaro Side Effects Start?
Side effects from Mounjaro can start within the first few days of taking your initial injection, though timing varies between individuals. Most people experience nausea or stomach-related effects during the first week, particularly when starting the 2.5 mg dose. Understanding when these effects typically appear helps you prepare and know what to expect as you begin treatment. This guide covers the timeline, severity patterns, and practical management strategies for UK adults taking this prescription medication.
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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.
Timeline of Mounjaro Side Effects by Dose
Starting dose (2.5 mg): Side effects often appear within 24–72 hours of injection. Nausea is the most common, typically peaking in days 2–3. Gastrointestinal symptoms like diarrhoea or constipation may develop 2–4 days after injection.
Escalation doses (5 mg onwards): When you increase your dose after 4 weeks, side effects may temporarily return or intensify for the first few days, then often settle as your body adjusts. This pattern can repeat with each dose increase on your tirzepatide escalation schedule.
- Most side effects peak between days 2–5 after injection
- Many people report improvement by day 6–7
- Effects vary significantly between individuals
- Some side effects (like appetite changes) may persist longer
If side effects are severe or prolonged beyond the first week, contact your prescriber before your next dose.
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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.
Managing Side Effects in the First Week
During the first few days after injection, eat smaller, frequent meals and avoid fatty or spicy foods, which often worsen nausea. Stay hydrated with water and electrolyte drinks. If nausea is severe, ask your GP or pharmacist about anti-nausea medication options. Keep a simple progress tracker to log when symptoms appear and how long they last — this helps you recognise patterns and manage future doses more confidently.
When to contact your prescriber: Seek urgent advice if you experience severe, persistent stomach pain (possible pancreatitis), signs of dehydration, or symptoms that don't improve after 7 days.
- 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.
Do Mounjaro side effects get worse with each dose increase?
Not always. Many people experience a temporary return of side effects (especially nausea) when their dose increases, but these often settle within a few days as the body adjusts. Some find side effects improve overall as they reach higher doses. Each person's experience is different, which is why monitoring your response is important.
How long do Mounjaro side effects last overall?
For most people, the initial acute side effects (nausea, diarrhoea) settle within 5–7 days of injection. However, mild effects may persist between doses. After 4–8 weeks of treatment, many report that side effects decrease significantly as their body adapts. This varies widely between individuals.
Can I take anything to prevent Mounjaro side effects?
Your GP or prescriber can recommend anti-nausea medication if needed. Lifestyle adjustments — small meals, avoiding fatty foods, staying hydrated — help many people. Starting on the lowest dose (2.5 mg) and escalating slowly, as recommended, reduces the intensity of initial side effects.
Are side effects a sign Mounjaro isn't working?
No. Side effects don't indicate whether Mounjaro will help with weight loss — they're simply your body's response to the medication. Many people experience both side effects and meaningful weight loss. Your prescriber will assess effectiveness based on your response over weeks and months, not just early side effects.
Should I stop Mounjaro if side effects start immediately?
Don't stop without contacting your prescriber first. Mild to moderate side effects in the first few days are common and often settle naturally. However, if you experience severe pain, persistent vomiting, or signs of an allergic reaction, seek urgent medical attention and discuss with your GP whether continuing is appropriate.
Ready to take control of your weight loss journey?
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