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 for Weight Loss — What to Expect
Mounjaro (tirzepatide) is a prescription-only weekly injection that may help with weight loss, but like all medications it can cause side effects. Understanding what to expect helps you make an informed decision about whether this weight loss injection is right for you. Side effects range from mild nausea to more serious complications, though most are temporary. This guide explains the most common and serious side effects reported in clinical trials, what you should monitor for, and when to contact your GP. Always discuss potential risks with your prescriber before starting any prescription weight loss treatment.
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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.
Most Common Mounjaro Side Effects
Clinical trials show that nausea, diarrhoea, vomiting, and constipation are the most frequently reported side effects of Mounjaro. These gastrointestinal effects occur because tirzepatide slows how quickly food moves through your stomach (gastric emptying), which your body typically adjusts to over time. Nausea affects more than 1 in 10 people in trials and often improves as your body adapts to the medication.
Other very common side effects include:
- Stomach or abdominal pain
- Indigestion and acid reflux
- Injection site reactions (redness, bruising, mild pain)
- Fatigue and tiredness
- Dizziness
- Hair loss (alopecia)
- Decreased appetite
Most of these effects are mild to moderate and tend to improve within the first few weeks as your body adjusts to the medication. If side effects persist or worsen, inform your GP or prescriber immediately.
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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
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Serious Side Effects — When to Seek Help
While rare, Mounjaro can cause serious side effects requiring urgent medical attention. Severe, persistent stomach pain spreading to your back may indicate pancreatitis (pancreas inflammation) — contact NHS 111 or A&E immediately if this occurs. Other serious warning signs include sudden vision changes (possible diabetic retinopathy complications if you have diabetes), severe allergic reactions (swelling of face, lips, or throat), and signs of gallbladder problems (pain in the upper right stomach area).
Additionally, Mounjaro may affect how other oral medications work, including oral contraceptives. Women using birth control pills should add barrier contraception for 4 weeks after starting and after each dose increase. Always inform your GP or prescriber of all medications and medical conditions before beginning treatment. Use our free GLP-1 weight loss calculator to understand projected outcomes, and track your response with our progress tracker.
- 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.
Is nausea from Mounjaro permanent?
No. Nausea is very common initially (affecting more than 1 in 10 people) but typically improves within 2–4 weeks as your body adjusts. Starting at the lowest dose (2.5 mg) and increasing gradually helps minimise nausea. Taking small, frequent meals and staying hydrated can also help. If nausea persists beyond 4 weeks, contact your prescriber.
Can Mounjaro side effects be managed?
Yes. Many side effects improve naturally with time. For gastrointestinal effects, eat smaller meals, avoid fatty or spicy foods, and drink plenty of water. Anti-sickness medication may be prescribed if nausea is severe. Your GP or prescriber can offer additional strategies based on your specific side effects.
Who should not take Mounjaro due to side effects risk?
Mounjaro is not suitable for anyone with a personal or family history of thyroid cancer, multiple endocrine neoplasia type 2, or severe pancreatitis. Pregnant or breastfeeding women should not use it. Anyone with a history of gallbladder problems needs careful assessment. Your prescriber will review your medical history during your clinical assessment.
What's the difference between Mounjaro and Wegovy side effects?
Mounjaro (tirzepatide) and Wegovy (semaglutide) both cause similar side effects, but clinical data suggests Mounjaro may have slightly higher rates of gastrointestinal effects initially, partly due to its dual mechanism of action. Individual tolerance varies significantly. Your prescriber can discuss which is more suitable for you based on your health profile.
Should I stop Mounjaro if I experience side effects?
Do not stop suddenly without consulting your prescriber. Most side effects improve naturally. Contact your GP or prescriber if side effects are severe or persistent — they may adjust your dose, offer management strategies, or switch you to an alternative medication. Always seek urgent care (NHS 111 or A&E) for serious symptoms like severe stomach pain or allergic reactions.
Ready to take control of your weight loss journey?
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