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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Tirzepatide Side Effects — What You Need to Know
Tirzepatide (brand name Mounjaro) is a prescription-only weekly injection licensed in the UK for weight management. Like all medications, tirzepatide can cause side effects, ranging from mild to serious. Most side effects are gastrointestinal — nausea, diarrhoea, and constipation — and tend to improve as your body adjusts. Understanding what to expect helps you make an informed decision with your GP about whether this medication is right for you. This guide covers the full range of tirzepatide side effects, what the NHS says about them, and when to seek medical attention.
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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 Tirzepatide Side Effects and What to Expect
Tirzepatide works by mimicking a GLP-1 receptor agonist and GIP receptor agonist — a dual mechanism that slows stomach emptying and reduces appetite. This action frequently causes gastrointestinal side effects, especially when first starting or increasing doses.
Very common side effects (more than 1 in 10 people):
- Nausea — often most noticeable in weeks 1–4 and typically improves with time
- Diarrhoea — usually mild to moderate
- Vomiting — less common than nausea but possible
- Constipation — can alternate with diarrhoea
- Stomach pain — typically mild abdominal discomfort
Common side effects (up to 1 in 10 people):
- Indigestion and acid reflux
- Injection site reactions (redness, bruising, mild pain)
- Fatigue and tiredness
- Dizziness
- Hair loss (alopecia) — reversible
- Decreased appetite — intentional but sometimes pronounced
Most side effects are mild to moderate and improve within a few weeks as your body adjusts to the medication. Your prescriber may recommend managing symptoms through dietary changes — eating smaller, frequent meals and staying hydrated helps significantly. If side effects persist or worsen, always contact your healthcare provider before stopping the medication.
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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 and When to Seek Help
While rare, some tirzepatide side effects require urgent medical attention. Seek immediate help if you experience:
- Severe stomach pain spreading to your back — possible pancreatitis, a serious inflammation of the pancreas
- Persistent vomiting — risk of dehydration
- Serious allergic reaction — swelling of face, lips, tongue, or throat, difficulty breathing
- Changes in vision — particularly important if you have diabetes, as GLP-1 medications can affect diabetic retinopathy
- Gallbladder problems — pain in the upper stomach, persistent nausea after meals
- Rapid heartbeat or chest pain — seek emergency care immediately
These serious side effects affect fewer than 1 in 100 people, but they require medical evaluation. If you develop any of these symptoms, contact your GP or NHS 111 immediately. Never stop tirzepatide suddenly without medical guidance — your prescriber will advise the safest way to manage your treatment if serious side effects occur.
- 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's the difference between tirzepatide side effects and Wegovy side effects?
Both are GLP-1-based medications, but tirzepatide is a dual GIP and GLP-1 receptor agonist, whereas Wegovy (semaglutide) is GLP-1 only. Tirzepatide may cause slightly more gastrointestinal side effects due to its dual mechanism, but the types of side effects are similar. Individual responses vary considerably. Your prescriber will help you choose based on your medical history.
How long do tirzepatide side effects last?
Most common side effects (nausea, diarrhoea) improve significantly within 2–4 weeks as your body adjusts. More persistent side effects like hair loss or fatigue may take longer to resolve. If side effects don't improve after 4 weeks, contact your prescriber — they may adjust your dose or recommend management strategies.
Can I take tirzepatide if I have a history of pancreatitis?
No. Tirzepatide is contraindicated (not suitable) if you have a personal or family history of pancreatitis or medullary thyroid cancer. You must disclose your full medical history to your prescriber before starting. If you have any concerns about your eligibility, discuss this with your GP.
Are tirzepatide side effects worse at higher doses?
Side effects can be more pronounced when starting or increasing doses. The standard dosing schedule (starting at 2.5 mg and increasing gradually each week) is designed to minimise side effects. Most people tolerate maintenance doses (10–15 mg) better than initial doses because their body has adjusted. If side effects worsen, your prescriber may slow your dose escalation.
What should I do if I experience nausea with tirzepatide?
Mild nausea typically improves without intervention. Eat smaller, frequent meals, avoid fatty or greasy foods, stay hydrated, and take your injection at the same time weekly. If nausea is severe or persistent after 4 weeks, contact your prescriber — they may recommend anti-nausea medication or adjust your dose schedule.
Ready to take control of your weight loss journey?
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