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 is a prescription-only GLP-1 and GIP receptor agonist medication available in the UK as Mounjaro for weight management. Like all medicines, tirzepatide can cause side effects. Most are mild and temporary, occurring as your body adjusts to the weekly weight loss injection. Understanding what to expect helps you manage treatment effectively and know when to contact your GP. This guide covers common side effects, serious reactions, and practical management strategies for UK adults.
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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 Tirzepatide
The most frequently reported tirzepatide side effects affect the digestive system, particularly when starting treatment or increasing your Mounjaro dose. Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These typically occur in the first few weeks as your body adjusts to the weekly injection.
Other common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux, fatigue, dizziness, headache, and injection site reactions such as redness or bruising. Hair loss has also been reported. Most side effects tend to improve within 2–4 weeks as your body adapts. If nausea persists, eating smaller, blander meals and staying hydrated can help. Use our free weight loss calculator to track your progress despite any side effects you experience.
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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, seek urgent medical attention if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), gallbladder problems with pain in the upper stomach area, serious allergic reactions including swelling of the face or throat, or unexplained changes in vision. These require immediate GP assessment or A&E attendance.
Additionally, if you experience rapid or unusual changes in heart rate, severe dehydration, or signs of diabetic complications if you have diabetes, contact your GP promptly. Your prescriber will have reviewed your medical history before issuing your Mounjaro prescription, but always report any new symptoms. Our BMI calculator helps you monitor health context alongside medication use.
Managing Side Effects and Treatment DecisionsMost tirzepatide side effects are manageable with practical strategies. For nausea, try eating slowly, avoiding fatty or greasy foods, and staying upright after meals. For constipation, increase fibre and water intake gradually. Dizziness may improve by standing up slowly from sitting or lying down. Injection site reactions usually fade; rotating injection sites (abdomen, thigh, upper arm) helps prevent buildup.
If side effects are severe or don't improve within 4 weeks, contact your GP or prescriber—they may adjust your dose or suggest alternative treatments. Remember, tirzepatide is a prescription-only medication and your prescriber can only assess your suitability through clinical consultation. Track your weight loss journey alongside any side effects using our free progress tracker to share with your healthcare team.
- 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 tirzepatide side effects last?
Most side effects improve within 2–4 weeks as your body adjusts. Nausea and digestive issues are typically temporary. If side effects persist beyond 4 weeks or worsen, contact your GP or prescriber for guidance. Dose adjustments may help.
Can I take tirzepatide if I have a sensitive stomach?
If you have a history of stomach problems, inform your prescriber before starting tirzepatide. A clinical assessment will determine suitability. Your GP can discuss risks versus benefits. Some people with digestive conditions manage tirzepatide successfully with dose adjustments.
Is nausea with tirzepatide a sign it's working?
Nausea doesn't necessarily indicate effectiveness—it's a side effect as your body adapts to the GIP receptor and GLP-1 receptor agonist mechanism. Weight loss occurs even without nausea. If nausea is severe, speak to your prescriber about management strategies or dose timing.
What should I do if I experience severe stomach pain on tirzepatide?
Severe stomach pain spreading to your back requires urgent medical attention as it may indicate pancreatitis. Contact your GP immediately or attend A&E. Do not delay. Your prescriber will have assessed your risk factors before prescription, but new symptoms must be evaluated promptly.
Can side effects return when increasing my tirzepatide dose?
Yes, some side effects may return temporarily when increasing from one Mounjaro dose to the next. This is normal as your body adjusts. Most settle within 1–2 weeks. Your prescriber spaces dose increases (minimum 4 weeks apart) to minimise this. Contact them if side effects are severe.
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.