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
Free tracker included with every download this week
What Are the Side Effects of Mounjaro 5mg?
Mounjaro 5mg is the first maintenance dose of tirzepatide, a prescription-only GLP-1 and GIP receptor agonist injection for weight management in the UK. Most people experience some side effects when starting or adjusting their dose, but these typically improve over time as your body adjusts to the medication. Understanding what to expect helps you manage symptoms and decide whether this treatment is right for you. Use our free BMI calculator to check if you may be eligible for this medication, and try our GLP-1 weight loss calculator to see projected outcomes at different doses.
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Calculate Your BMI Instantly
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 at 5mg Dose
At the 5mg dose, nausea and gastrointestinal symptoms are the most frequently reported side effects. In clinical trials, these typically began within the first few days of injection and often improved within one to two weeks as your body adjusted. Common side effects affecting more than 1 in 10 people include:
- Nausea — the most common; usually improves by week 2–3
- Diarrhoea — often mild to moderate
- Constipation — can alternate with diarrhoea
- Stomach pain or indigestion — typically mild
- Injection site reactions — redness, bruising, or soreness at injection site
- Fatigue or tiredness — reported by some users
- Dizziness or headache — usually mild and transient
These symptoms are not signs the medication isn't working — they are expected as your body adapts. Most people tolerate the 5mg dose well once the initial adjustment period passes.
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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.
Managing Side Effects and When to Seek Help
To minimise side effects at 5mg, eat smaller portions of bland, low-fat foods and stay well hydrated. Avoid fatty or spicy meals, which can worsen nausea and digestive symptoms. If nausea is severe, your prescriber may recommend over-the-counter anti-nausea support. Most people do not need to stop taking Mounjaro due to side effects — they improve naturally over days to weeks.
Seek urgent medical attention if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, or throat), or changes in vision. Contact your GP if side effects persist beyond three weeks or significantly impact your daily life. Your prescriber can advise whether staying at 5mg longer before escalating is appropriate.
- 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 at the 5mg dose normal?
Yes, nausea is very common at the 5mg starting maintenance dose and affects many people in their first 1–2 weeks. It typically improves as your body adjusts. Eating smaller meals, staying hydrated, and avoiding fatty foods can help. If nausea persists beyond three weeks or is severe, contact your GP or prescriber.
How long do 5mg side effects usually last?
Most gastrointestinal side effects improve within 1–3 weeks as your body adjusts. Injection site reactions usually resolve within a few hours. Individual timelines vary, but the majority of people find side effects are manageable and temporary. Persistent symptoms should be discussed with your prescriber.
Can I stop taking Mounjaro if side effects are too much?
Do not stop without medical guidance. Contact your GP or prescriber first — they can advise whether to stay at 5mg longer, reduce your dose, or explore other options. Many side effects improve with time and practical strategies. Sudden stopping may not be appropriate depending on your individual circumstances.
What's the difference between Mounjaro and Wegovy side effects?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Both cause similar gastrointestinal side effects. Mounjaro users report comparable nausea and diarrhoea rates. Individual tolerance varies significantly. Your prescriber can discuss which is more suitable for you.
Are 5mg side effects a sign Mounjaro isn't working?
No, side effects are not related to effectiveness. They are your body adapting to the medication's action on appetite and digestion hormones. Many people with excellent weight loss results experience initial side effects that improve over weeks. Effectiveness and tolerability are separate outcomes — both improve with time and 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.