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 Kick In?
Mounjaro side effects can start within days of your first injection, though timing varies significantly between individuals. Most people experience their first symptoms within the first week or two of starting treatment. Understanding when side effects typically begin, how long they last, and what you can do to manage them will help you prepare for your weight management journey with 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
Within the first few days: Nausea is often the first side effect, frequently appearing within 24–72 hours of your initial 2.5 mg injection. This is one of the most common experiences reported by UK patients starting tirzepatide. Some people notice mild stomach discomfort or reduced appetite almost immediately.
Week 1–2: Gastrointestinal symptoms typically peak during this window. Diarrhoea, constipation, and indigestion become more noticeable as your body adjusts to the medication. Nausea may intensify or improve depending on individual factors.
Weeks 3–4: For many people, side effects begin to plateau or gradually improve as your body adapts to the dose. However, if you're scheduled to increase from 2.5 mg to 5 mg at week 5, symptoms may restart or worsen after the dose increase. This pattern typically repeats with each dose escalation during your tirzepatide dose titration schedule.
- Nausea: usually improves by weeks 3–4 on the same dose
- Diarrhoea/constipation: may persist longer, sometimes weeks 6–8
- Appetite suppression: often becomes more pronounced over time
- Injection site reactions: typically mild and resolve within hours to days
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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 the Side Effects Timeline
The key to managing side effect timing is pacing your food intake and staying hydrated. Eat smaller, more frequent meals rather than large portions, as your stomach empties more slowly on Mounjaro. Ginger tea, peppermint, and simple carbohydrates can help with nausea during the first week. If side effects are severe or don't improve after 2–3 weeks on the same dose, contact your GP or prescriber—they can adjust your treatment plan or provide additional support.
It's important to note that side effects vary considerably: some UK patients report mild symptoms for just a few days, while others experience nausea or digestive issues for several weeks. Individual factors including age, metabolism, stomach sensitivity, and other medications all influence your personal timeline. Using our free GLP-1 weight loss calculator can help you understand realistic outcomes while managing expectations about the adjustment period.
- 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 I prevent Mounjaro side effects before they start?
You cannot eliminate side effects entirely, but starting with the lowest dose (2.5 mg) and eating small, frequent meals before your injection helps minimise symptoms. Staying well hydrated and avoiding fatty or spicy foods during your first week also reduces nausea severity. Most importantly, discuss any concerns with your prescriber before starting—they may adjust your approach based on your medical history.
Do side effects get worse when the dose increases?
Yes, often. When you increase from one dose to the next (e.g., 2.5 mg to 5 mg at week 5), you may experience a temporary return of side effects. This is normal and typically lasts 1–3 weeks before improvement. Some patients find their symptoms are milder on the second dose increase because their body has already adapted to the medication.
When should I contact my GP about Mounjaro side effects?
Contact your GP urgently if you experience severe, persistent stomach pain (possible pancreatitis), serious allergic reactions, or gallbladder symptoms. For regular side effects like nausea or diarrhoea, contact your prescriber if symptoms don't improve after 3–4 weeks on the same dose or are severely affecting your quality of life. They can advise on management strategies or dose adjustments.
How does Mounjaro's side effect timeline compare to Wegovy?
Both are GLP-1-based medications, so side effect profiles are similar. However, Mounjaro uses a dual GIP + GLP-1 mechanism and typically requires fewer dose escalations to reach the maximum effective dose. Wegovy has a slower titration schedule, which may spread side effects over a longer timeline. Individual responses vary significantly, regardless of which medication you're prescribed.
Will side effects eventually disappear completely?
For most people on a stable dose, gastrointestinal side effects significantly improve or resolve by weeks 4–8. Some patients report side effects lasting longer, particularly constipation or subtle appetite changes. Nausea is typically the shortest-lived symptom. However, if you remain on Mounjaro long-term, some appetite suppression usually persists—this is actually the intended therapeutic effect that supports weight management.
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.