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
How Long After Injecting Mounjaro Do Side Effects Start?
Most people experience Mounjaro side effects within the first few hours to days after their injection, though the timing varies considerably between individuals. The most common side effects—nausea, diarrhoea, constipation, and stomach discomfort—typically appear within 24–48 hours of injection. However, some people report side effects starting immediately or within the first few hours, whilst others may not experience them at all, especially at lower doses during the initial dose titration phase. Understanding this timeline helps you prepare and know when to contact your GP if symptoms become concerning.
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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.
Mounjaro Side Effects Timeline — When They Appear
Hour 0–6 after injection: Some people experience nausea, dizziness, or mild stomach discomfort within the first few hours. This is less common but can happen, particularly if you eat large meals shortly after injection.
24–48 hours: This is the most common window for side effects to emerge. Nausea, diarrhoea, and stomach pain are particularly likely during this period. Many people find symptoms are at their worst on the day after injection.
3–7 days after injection: For some individuals, side effects persist throughout the week but often begin to improve as the body adjusts. Constipation may develop later in the week than nausea.
Dose titration impact: Starting on a lower Mounjaro dose (2.5 mg) usually means milder side effects with a shorter duration. As your dose increases, side effects may temporarily worsen before your body adapts. Each dose increase follows at least a 4-week interval, allowing adjustment time between escalations.
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
Most Mounjaro side effects are manageable with simple strategies: eat smaller, lighter meals; stay hydrated; and avoid greasy or spicy foods. Many people find symptoms improve significantly after 2–3 weeks as their body adapts. However, contact your GP immediately if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), signs of a serious allergic reaction, or gallbladder symptoms. You don't need to stop your injection—just let your prescriber know. Using our free GLP-1 weight loss calculator can help you track projected results alongside managing side effects, and our progress tracker lets you log how you're feeling week by week, which is useful information to share with your GP.
- 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.
Can Mounjaro side effects start before the full 7 days are up?
Yes, absolutely. Most people experience side effects within 24–48 hours of injection. Some report symptoms within hours of injecting, whilst others don't experience side effects until day 3 or 4. If side effects appear early and are manageable, this is normal and typically improves over the following days.
Are side effects worse with higher Mounjaro doses?
Generally, yes. The 2.5 mg starting dose often causes mild side effects. As you progress through dose titration (5 mg, 7.5 mg, 10 mg, etc.), nausea and diarrhoea may temporarily intensify when you first increase the dose, but usually settle after a few days. This is why NICE and NHS guidance recommends waiting at least 4 weeks between dose increases.
What if I have no side effects from Mounjaro—is it still working?
Yes. Not everyone experiences side effects, especially at lower doses during the initial weeks. Side effects are not a measure of whether the medication is effective. Your weight loss, appetite changes, and how you feel are better indicators of whether tirzepatide is working for you. Discuss any concerns with your prescriber.
Should I stop taking Mounjaro if side effects start within hours of injection?
No—don't stop without speaking to your GP or prescriber first. Early-onset side effects are usually temporary and manageable. However, if you experience severe symptoms like extreme abdominal pain, difficulty breathing, or signs of an allergic reaction, seek medical attention immediately. For routine side effects, contact your GP for advice.
How do side effects from Mounjaro compare to Wegovy?
Both are GLP-1/GIP medications with similar side effect profiles (nausea, diarrhoea, constipation). Mounjaro's dual GIP and GLP-1 mechanism may cause slightly different symptom patterns in some people, and its weekly dosing differs from Wegovy's escalation schedule. Individual experiences vary widely—some people tolerate one better than the other. Your prescriber can discuss which is more suitable for you.
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.