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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What Are the Symptoms and Side Effects of Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight management injection that works by activating GLP-1 and GIP hormone pathways. Like all medications, it can cause side effects — though not everyone experiences them. Understanding what symptoms to expect helps you use the medication safely and manage any effects that do occur. This guide covers the most common symptoms, when they typically appear, and what to do if you're concerned.
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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 Mounjaro Symptoms and Their Timing
The most common side effects of Mounjaro affect the digestive system. Nausea is reported by more than 1 in 10 people and is often the first symptom experienced, typically appearing within the first week of starting or increasing your dose. Diarrhoea and constipation can occur as your body adjusts to the medication — some people experience both at different times. Stomach pain and indigestion are also common, especially after eating larger meals.
- Nausea: usually improves after 3–5 days as your body adjusts
- Diarrhoea: tends to settle within 1–2 weeks
- Constipation: manage with increased water and fibre intake
- Stomach pain: often linked to eating too quickly or too much
- Decreased appetite: an intended effect, but can feel extreme initially
Less common but important symptoms include fatigue, dizziness, headaches, and hair loss (alopecia). Injection site reactions — redness, bruising, or mild pain where you inject — typically resolve within a few hours. These symptoms usually reduce significantly after your body has adjusted, typically within 2–3 weeks at each new dose.
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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.
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Accessing Mounjaro in the UK: NHS vs Private Routes
In the UK, you can access Mounjaro through two main routes. NHS access is through specialist tier 3 weight management services following a GP referral. NICE has approved tirzepatide for use within NHS services, but availability varies significantly by region and waiting lists can be lengthy. Private access is through online consultations with UK-licensed prescribers, typically costing between £129–£320 per month depending on your dose.
The online consultation process involves completing a health questionnaire, having your eligibility assessed, and receiving a prescription if appropriate. A GPhC-registered pharmacy then dispenses your medication. Whether you choose NHS or private access, a qualified healthcare professional will assess whether Mounjaro is suitable for you and discuss potential symptoms before you start treatment.
- 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.
When do Mounjaro side effects usually start?
Most side effects appear within the first week of starting Mounjaro or increasing your dose. Nausea is typically the first symptom. Most effects are mild and tend to improve within 2–3 weeks as your body adjusts. If symptoms persist or worsen, contact your GP or prescriber for guidance.
Are Mounjaro symptoms the same as Wegovy symptoms?
Both Mounjaro and Wegovy cause similar digestive side effects — nausea, diarrhoea, and constipation are common to both. However, Mounjaro is a dual GIP and GLP-1 agonist whilst Wegovy is a GLP-1 agonist only, so some people may experience slightly different symptom patterns. Individual responses vary significantly.
How can I manage nausea from Mounjaro?
Eat smaller, more frequent meals; avoid fatty or greasy foods; stay hydrated; eat slowly; and avoid strong smells. Many people find nausea improves within 3–5 days. If severe or persistent nausea affects your daily life, speak to your prescriber — they may adjust your approach or suggest supportive measures.
When should I seek urgent medical help with Mounjaro symptoms?
Seek immediate medical attention if you experience severe stomach pain (especially pain that spreads to your back), serious allergic reactions (swelling of face, lips, or throat), or changes in vision. These may indicate pancreatitis or other serious complications requiring urgent assessment.
Can I use the MyWeightCheck GLP-1 calculator if I'm experiencing Mounjaro symptoms?
Yes, the weight loss calculator on this page can help you estimate projected outcomes even if you're managing side effects. However, if symptoms are severe, speak to your prescriber first before adjusting your expectations around weight loss or dose timing.
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.