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 5mg Side Effects — What You Should Know
Mounjaro 5mg is the first maintenance dose of tirzepatide, a prescription-only weight management injection licensed in the UK. Most people experience some side effects when starting Mounjaro, particularly nausea and gastrointestinal changes. Understanding what's normal, what's temporary, and when to seek help from your GP can make the treatment experience smoother. This guide covers the most common side effects at the 5mg dose, how long they typically last, and practical tips 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.
Most Common Side Effects at 5mg Dose
Clinical data shows that the majority of people taking Mounjaro 5mg experience gastrointestinal side effects, particularly during the first 4 weeks at this dose. Nausea is the most frequently reported side effect, affecting up to 1 in 3 people, though it typically improves within 1–2 weeks as your body adjusts. Diarrhoea and constipation both occur commonly — sometimes alternating — as Mounjaro slows stomach emptying. Stomach pain or mild discomfort is also very common. Other frequent effects include indigestion, reduced appetite (which many users find helpful for weight management), fatigue, and mild dizziness.
- Nausea — usually improves within 1–2 weeks
- Diarrhoea or constipation — may persist longer
- Stomach pain — typically mild and temporary
- Reduced appetite — often considered a benefit
- Fatigue or dizziness — usually temporary
Most side effects at 5mg are mild to moderate and reduce as your body adjusts to the medication. Individual experiences vary significantly.
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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 Contact Your GP
While most side effects of Mounjaro 5mg are mild and temporary, some symptoms require immediate medical attention. Severe, persistent stomach pain spreading to your back could indicate pancreatitis and needs urgent evaluation. Gallbladder problems — including pain in the upper stomach area, jaundice (yellowing of skin or eyes), or dark urine — require GP assessment. Serious allergic reactions (swelling of face, lips, tongue, or throat) are rare but medical emergencies. Contact your GP or NHS 111 if you experience any of these. You should also tell your prescriber about any persistent side effects affecting your quality of life — alternative doses or medications may be options.
- 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 5mg side effects last?
Most side effects improve within 1–2 weeks as your body adjusts. Nausea typically resolves quickly, though gastrointestinal changes may take longer. If side effects persist beyond 2–3 weeks, contact your GP — adjusting your dose or taking the injection at a different time may help.
Can I manage nausea while taking Mounjaro 5mg?
Yes. Eat smaller, more frequent meals; avoid fatty or greasy foods; stay hydrated; take ginger supplements; and eat slowly. Taking Mounjaro at bedtime may help reduce daytime nausea. Ask your prescriber before taking anti-nausea medication, as some may interact with Mounjaro.
Is Mounjaro 5mg the same as the starting dose?
No. Mounjaro starts at 2.5mg for weeks 1–4, then increases to 5mg for weeks 5–8. At 5mg, your body is already partially adjusted, so side effects are often less intense than at the starting dose, though they can still occur.
Should I stop Mounjaro if I get side effects at 5mg?
Do not stop without speaking to your prescriber first. Most side effects improve naturally. Stopping suddenly may prevent you from achieving weight loss results. Your prescriber can suggest practical management strategies or adjust your plan if needed.
How does Mounjaro 5mg compare to Wegovy for side effects?
Both are GLP-1-based injections causing similar side effect profiles. Mounjaro uses a dual GIP+GLP-1 mechanism, while Wegovy (semaglutide) is GLP-1-only. Some people report slightly different tolerability, but individual variation is larger than medication differences. Your GP or prescriber can help you choose based on your needs.
Ready to take control of your weight loss journey?
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