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
Side Effects of Mounjaro Injections
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. Like all medicines, it can cause side effects. Most are mild and improve over time as your body adjusts, though some people experience no side effects at all. Understanding what to expect helps you decide if this treatment is right for you. Always discuss potential side effects with your GP or prescriber before starting, especially if you have existing health conditions. This guide covers the most common and serious side effects reported during UK clinical use.
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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.
Common Side Effects of Mounjaro
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. Nausea is the most frequently reported side effect, particularly when starting Mounjaro or increasing your dose. These gastrointestinal effects typically improve within a few weeks as your body adjusts to the tirzepatide. Many people find that eating smaller meals, staying hydrated, and avoiding fatty foods helps reduce nausea. Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux, injection site reactions (redness or bruising), fatigue, dizziness, and hair loss. Injection site reactions are usually mild and resolve within a few days. If constipation becomes severe, increase fibre and water intake or speak to your GP about laxatives. Most side effects are manageable and do not require stopping treatment.
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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.
When to Seek Urgent Medical Attention
Stop taking Mounjaro and seek immediate medical help if you experience severe, persistent stomach pain spreading to your back (possible sign of pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), signs of gallbladder problems (upper stomach pain, yellowing of skin or eyes), or unexplained changes in vision. If you have a personal or family history of medullary thyroid cancer, you should not take Mounjaro. Always inform your prescriber of any unusual or worsening symptoms. Your GP can provide personalised advice based on your individual health. Use our free BMI calculator and GLP-1 weight loss calculator above to explore whether Mounjaro might be suitable for you before discussing with a healthcare professional.
- 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.
What is the most common side effect of Mounjaro?
Nausea is the most frequently reported side effect, especially when starting treatment or increasing your dose. It typically improves within a few weeks as your body adjusts. Eating smaller, low-fat meals and staying hydrated can help manage it. If nausea persists beyond 4 weeks, speak to your prescriber about whether continuing is right for you.
How long do Mounjaro side effects last?
Most common side effects improve within 2–4 weeks as your body adapts to the tirzepatide. Gastrointestinal symptoms like nausea and diarrhoea are usually temporary. If side effects persist beyond 4 weeks or are severe, discuss with your GP whether you should continue, switch doses, or try a different medication.
Can you stop Mounjaro if side effects are too much?
Yes. If side effects are severe or unbearable, you can stop Mounjaro. Speak to your prescriber first rather than stopping abruptly. You may return to your previous dose if a higher dose causes problems, or your prescriber may recommend a different weight loss medication like Wegovy (semaglutide). Never change your dose without clinical guidance.
Is hair loss a permanent side effect of Mounjaro?
Hair loss (alopecia) has been reported as a common side effect during treatment. In most cases, hair loss is temporary and resolves after stopping Mounjaro or as your body adjusts. If you experience noticeable hair loss, inform your GP. This side effect is less common than gastrointestinal effects and does not occur in everyone taking the medication.
What's the difference between Mounjaro and Wegovy side effects?
Both are GLP-1-type medications causing similar side effects (nausea, diarrhoea, constipation), but Mounjaro also activates the GIP pathway, which may cause different tolerability for some people. Mounjaro doses are titrated weekly over several weeks, while Wegovy escalates over 16 weeks—potentially different side effect timelines. Individual experiences vary greatly; what affects one person may not affect another.
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