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?
Most people taking Mounjaro experience side effects within the first few days after their initial injection, though timing and severity vary significantly between individuals. The most common side effects—nausea, diarrhoea, and stomach discomfort—typically emerge within 24 to 48 hours of taking your first dose. Understanding this timeline and knowing what to expect can help you manage symptoms effectively and stay on track with your treatment plan under NHS or private prescription guidance.
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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.
Side Effect Timeline and What to Expect
Most side effects appear within the first 1–3 days after your Mounjaro injection. Nausea is the most frequently reported early symptom, often emerging within 24 hours. Other gastrointestinal effects—including diarrhoea, constipation, vomiting, and abdominal discomfort—typically develop within the same timeframe but may persist longer.
Importantly, side effects often improve within the first week to 10 days as your body adjusts to the tirzepatide medication. However, when you increase to a higher dose (moving through the dose titration schedule from 2.5 mg to 5 mg to 10 mg and beyond), you may experience a temporary return of side effects similar to your initial injection.
- Days 1–3: Peak nausea and GI upset typically occur
- Days 4–7: Symptoms often begin to settle as your body adapts
- Week 2 onwards: Many people report significant improvement
- Each dose increase: Temporary symptom return is common but usually resolves quickly
Clinically BackedWeight Loss
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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 and When to Seek Help
If you experience severe nausea or vomiting that prevents you from eating or drinking, contact your GP or prescriber immediately. Persistent stomach pain spreading to your back may indicate pancreatitis and requires urgent medical attention. For milder symptoms, eating small, frequent meals, staying hydrated, and avoiding fatty or spicy foods can help significantly.
Your prescriber may adjust your dose schedule or provide additional guidance if side effects are severe. Most people find their symptoms manageable within 1–2 weeks, allowing them to continue benefiting from Mounjaro's weight management effects. Keep a symptom diary to help your healthcare provider understand your individual response and optimise your treatment plan.
- 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 side effects happen with every Mounjaro dose increase?
Most people experience a temporary return of side effects—particularly nausea—when moving to a higher dose during their dose titration schedule. However, these typically resolve within a few days, just as with the initial injection. Your body adapts more quickly the second time. Inform your prescriber if side effects are severe so they can provide additional support.
Is it normal to feel nauseous for a whole week after Mounjaro?
Nausea lasting a full week is less common but not unusual, especially at higher doses. Most people see improvement within 3–5 days. If nausea persists beyond 10 days or is severe, contact your GP or prescriber. They may suggest dietary adjustments, anti-nausea medication, or a slower dose increase schedule to help manage symptoms.
What's the difference between Mounjaro and Wegovy side effect timelines?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) typically cause side effects within 1–3 days of injection. Mounjaro's dual GIP/GLP-1 mechanism may cause slightly different GI effects than Wegovy's single GLP-1 pathway, but onset timing is similar. Individual experiences vary significantly regardless of which medication you're prescribed.
Can I take anti-nausea medication while on Mounjaro?
Yes, over-the-counter anti-nausea remedies like ginger or anti-emetic tablets are generally safe alongside Mounjaro, but always inform your prescriber or GP first. They can recommend options suitable for you and rule out any interactions with your other medications. Your UK prescriber or GP is your best source for personalised guidance.
Should I stop Mounjaro if side effects start immediately?
No—side effects starting within 24–48 hours are expected and normal, not a reason to stop. Most people find symptoms improve significantly within a week. However, if you experience severe symptoms (severe vomiting, severe abdominal pain, or signs of allergic reaction), contact your GP or prescriber immediately rather than stopping on your own.
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.