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
What Are Mounjaro Side Effects?
Mounjaro, often called a "weight loss injections" in the UK, is a prescription-only once-weekly injection containing tirzepatide. Like all medications, it can cause side effects. Most side effects are mild and improve over time, particularly during the dose titration phase. Understanding what to expect helps you and your prescriber manage your weight loss journey safely and effectively.
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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
The most frequently reported side effects of Mounjaro affect the digestive system. Nausea is very common, especially in the first few weeks, and typically improves as your body adjusts. Diarrhoea, constipation, and stomach pain are also very common. Vomiting occurs in some users, particularly at higher doses.
Other common side effects include indigestion, acid reflux, fatigue, dizziness, injection site reactions (redness or bruising), and hair loss (alopecia). Most of these effects are mild to moderate and tend to settle as you progress through the mounjaro dose titration schedule. Your prescriber may suggest lifestyle adjustments—like eating smaller, slower meals or staying hydrated—to manage digestive symptoms.
- Nausea and vomiting (often improve after 2–4 weeks)
- Diarrhoea or constipation
- Stomach pain or indigestion
- Fatigue and dizziness
- Injection site reactions
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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.
Serious Side Effects — When to Seek Help
While serious side effects are uncommon, you must seek immediate medical attention if you experience severe, persistent stomach pain that spreads to your back (possible pancreatitis), signs of a serious allergic reaction (swelling of face, lips, tongue, or throat), changes in vision, or gallbladder pain (pain in the upper stomach area).
Mounjaro may also increase heart rate in some people. If you notice unusual heart palpitations or chest discomfort, contact your GP or NHS 111 immediately. Because Mounjaro slows stomach emptying, it's not suitable for people with severe gastroparesis or digestion problems. Always report any concerning symptoms to your prescriber—they can assess whether to adjust your dose or consider an alternative.
- 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.
Are Mounjaro side effects permanent?
Most side effects are temporary and improve within 2–4 weeks as your body adjusts. Digestive symptoms (nausea, diarrhoea) are most common early on. If side effects persist or worsen, contact your prescriber—they may adjust your dose or discuss alternatives. Some effects, like hair loss, may resolve after stopping the medication.
Can I reduce Mounjaro side effects?
Yes. Eat smaller, slower meals; stay well hydrated; avoid high-fat or spicy foods; and get adequate sleep. Ginger tea or anti-nausea remedies may help. Your prescriber can also pause dose increases if side effects are troublesome. The mounjaro KwikPen is easy to use, and consistent injection technique helps minimise site reactions.
How does Mounjaro compare to Wegovy for side effects?
Both are GLP-1-based medications with similar side effect profiles. Mounjaro is a dual GIP and GLP-1 agonist (stronger mechanism), so some users report slightly more nausea initially. Wegovy (semaglutide) is a GLP-1-only agonist. Individual experiences vary—your prescriber can discuss which is most suitable for you based on your mounjaro vs wegovy tolerance profile.
Will side effects stop me from losing weight?
Not necessarily. Clinical trials show most users continue treatment despite mild side effects because the benefits outweigh discomfort. Use our free GLP-1 weight loss calculator to see projected results at different doses—your prescriber may start at a lower dose and titrate slowly to minimise side effects while achieving your goals.
What should I tell my GP about side effects?
Report any persistent or concerning symptoms, especially severe stomach pain, vision changes, or allergic reactions. Your GP can assess whether your mounjaro UK prescription needs adjustment, consider whether you're eligible for mounjaro nhs nice recommendations, or explore alternatives. Never stop taking Mounjaro without medical guidance—always discuss with your prescriber first.
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.