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 Side Effects — What You Need to Know
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection that works by activating GLP-1 and GIP hormone pathways. Like all medications, it can cause side effects — though not everyone experiences them. Most side effects are mild to moderate and tend to improve over time as your body adjusts. Understanding what to expect helps you manage them effectively and decide whether this treatment is right for you. This guide covers the most common side effects reported in UK clinical trials and practical advice for managing them.
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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 Side Effects of Mounjaro
The most frequently reported side effects of Mounjaro affect the digestive system, particularly when starting the medication or increasing your dose. Nausea is very common in the first few weeks but typically improves as your body adjusts — usually within 2–4 weeks as the dose titration progresses. Diarrhoea and constipation can both occur and may alternate; staying hydrated and eating slowly can help manage these. Stomach pain and indigestion are common but usually mild. Vomiting affects some users, especially if you eat too quickly or too much at once. Other common effects include fatigue, dizziness, headache, and injection site reactions (redness or bruising). Hair loss has been reported but is typically temporary. Clinical trials showed these effects were more pronounced during dose titration — when your prescriber gradually increases your dose from 2.5 mg up to 15 mg over several months — and often settle as you stabilise on your maintenance dose.
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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 rare, some side effects require urgent medical attention. Contact your GP or call NHS 111 immediately if you experience severe persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), sudden changes in vision, or signs of gallbladder problems (severe pain in the upper stomach). These are uncommon but need prompt evaluation. Keep a record of any side effects you experience and discuss them at your regular prescriber reviews — usually every 4 weeks when adjusting doses. Most people find that starting at the lowest dose (2.5 mg) and increasing gradually minimises side effects. If side effects are severe or intolerable, your prescriber can pause dose increases or adjust your 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.
How long do Mounjaro side effects last?
Most side effects improve within 2–4 weeks as your body adjusts. Nausea and digestive issues typically settle once you stabilise on your maintenance dose. If side effects persist beyond 4 weeks, discuss this with your prescriber — you may need a slower dose increase or dietary adjustments.
Can I reduce Mounjaro side effects by eating differently?
Yes. Eating slowly, in smaller portions, and choosing bland foods (rice, toast, plain chicken) can reduce nausea and stomach discomfort. Avoid fatty, spicy, or high-fibre foods initially. Stay hydrated with water and electrolyte drinks. Your GP or a dietitian can offer personalised guidance.
Is nausea from Mounjaro the same as feeling full?
No — they're different. Mounjaro-related nausea is a medication side effect, while feeling fuller after smaller meals is the intended therapeutic effect. Nausea typically fades, but the appetite-reducing effect persists, supporting your weight loss goals.
Do all Mounjaro side effects mean I should stop taking it?
Not necessarily. Mild side effects often improve over time and may be worth tolerating for the weight loss benefits — your prescriber can help you weigh pros and cons. Serious side effects (severe pain, allergic reactions) require immediate medical attention and possible discontinuation.
Can I use our GLP-1 weight loss calculator to predict side effects?
Our GLP-1 weight loss calculator projects potential weight loss at different doses but cannot predict individual side effects — these vary significantly. Use it alongside discussions with your prescriber about what to expect and how to manage side effects during your treatment.
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.