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
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Mounjaro Injection Side Effects — What UK Patients Should Know
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection licensed in the UK for adults with obesity or overweight with related health conditions. Like all medicines, Mounjaro may cause side effects. Most are mild and improve over time as your body adjusts, but it's important to understand what to expect and when to contact your GP. This guide covers the common, uncommon, and serious side effects reported in clinical trials and UK practice, helping you make an informed decision about whether this weekly weight loss injection is right for you.
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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.
Common Mounjaro Side Effects — What Most People Experience
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These typically occur during dose escalation and often improve within a few days to weeks as your body adjusts to the medication. Nausea is the most frequently reported side effect and is usually mild to moderate.
Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux (GORD), injection site reactions such as redness or bruising, fatigue, dizziness, and hair loss. Most injection site reactions are minor and resolve quickly. Fatigue and dizziness are often manageable through hydration and rest.
Try our GLP-1 weight loss calculator to project your potential weight loss at different doses — understanding expected outcomes can help contextualise side effects as part of the treatment journey. Most people find side effects are a worthwhile trade-off for significant weight loss results.
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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.
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Serious Side Effects — When to Contact Your GP Immediately
Whilst serious side effects are rare, you should seek immediate medical attention if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), changes in vision (possible diabetic retinopathy), gallbladder problems (pain in the upper stomach area), or rapid heartbeat or chest pain.
Additionally, contact your GP if you experience signs of dehydration (extreme thirst, dark urine, dizziness when standing), severe or worsening nausea and vomiting preventing you from eating or drinking, or any unexpected symptoms. Your prescriber will have assessed your suitability for this weekly weight loss injection, but ongoing monitoring is important. Always inform your GP of all side effects, even mild ones, as they may affect your dose or 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.
What are the most common Mounjaro side effects in the UK?
The most common side effects are nausea, diarrhoea, vomiting, constipation, and stomach pain. These affect more than 1 in 10 people and typically improve within days to weeks as your body adjusts. Nausea is most frequent but usually mild to moderate and manageable with small, frequent meals.
How long do Mounjaro side effects last?
Most side effects improve significantly within 1–2 weeks of starting or increasing a dose. Nausea and digestive issues typically resolve as your body adapts. Some people experience persistent mild nausea, but this often improves at higher maintenance doses. Individual timelines vary.
Can I reduce side effects while taking Mounjaro?
Yes. Eating small, frequent meals, avoiding fatty or spicy foods, staying hydrated, and getting adequate rest can reduce nausea and digestive side effects. Your prescriber may adjust your dose or timing if side effects are severe. Never stop or reduce your dose without medical guidance.
Is hair loss a common Mounjaro side effect in the UK?
Hair loss (alopecia) is reported as a common side effect in the product information, but it is not experienced by everyone. gradual weight management itself can temporarily increase hair shedding. If hair loss concerns you, discuss it with your GP — it may warrant dose adjustment or additional support.
When should I contact my GP about Mounjaro side effects?
Contact your GP if you experience severe stomach pain (possible pancreatitis), serious allergic reactions, vision changes, gallbladder pain, rapid heartbeat, severe dehydration, or persistent vomiting preventing eating or drinking. Always report unusual symptoms — your GP can assess whether they require intervention or dose adjustment.
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.