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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Managing Upset Stomach on Mounjaro in the UK
An upset stomach is one of the most common side effects when starting Mounjaro (tirzepatide), affecting more than 1 in 10 people. If you're experiencing nausea, diarrhoea, or stomach discomfort while on this GLP-1 and GIP receptor agonist, you're not alone. This guide explains what causes digestive upset on Mounjaro, how to manage it, and what to know about accessing Mounjaro in the UK — whether through NHS specialist services or private prescription routes. Understanding both the side effects and your access options can help you make an informed decision about weight management treatment.
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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.
UK Mounjaro Access Routes and Prescription Costs
In the UK, Mounjaro is available through two main routes: NHS specialist weight management services and private prescription.
NHS Route: Mounjaro may be prescribed through tier 3 or tier 4 specialist weight management services if you meet eligibility criteria (BMI ≥30, or BMI 27–29.9 with weight-related health conditions). Access is free through the NHS, but availability varies significantly by region and waiting lists can be lengthy — sometimes months to over a year depending on your ICB (Integrated Care Board). Your GP can refer you to your local specialist service.
Private Route: Private prescription through UK-licensed online prescribers costs approximately £130–£320 per month depending on your dose. The Mounjaro KwikPen (containing 4 weekly doses) is dispensed by GPhC-registered pharmacies following an online clinical assessment. Private access does not require an NHS referral and treatment can typically begin within days. A prescriber will assess whether Mounjaro is suitable based on your health history, current medications, and weight management goals.
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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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Managing Digestive Side Effects During Dose Titration
Upset stomach is most common during the first 4–8 weeks and often improves as your body adjusts. Mounjaro dose titration starts at 2.5mg weekly, increasing every 4 weeks to allow your digestive system to adapt. Practical strategies include eating smaller, frequent meals; avoiding fatty or greasy foods; staying hydrated; and eating slowly. If nausea is severe, speak to your prescriber — they may recommend staying on your current dose longer before increasing, or switching to a different medication like Wegovy (semaglutide). Most side effects settle significantly by weeks 8–12. Use our free BMI calculator to understand your starting point and baseline weight, and our GLP-1 weight loss calculator to project realistic outcomes as you manage side effects. Many people find that as digestive symptoms ease, weight loss becomes more consistent and manageable.
- 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 does upset stomach last on Mounjaro?
Digestive upset is most common in the first 4–8 weeks. Most people report significant improvement by week 8–12 as their body adjusts. If symptoms persist or worsen, contact your prescriber — you may need to stay on your current dose longer before increasing, or consider an alternative like Wegovy.
What's the difference between NHS and private Mounjaro in the UK?
NHS Mounjaro is free but requires referral to specialist services with potential waiting lists. Private Mounjaro costs £130–£320/month and is faster to access (days rather than months), prescribed following an online clinical assessment by a UK-licensed prescriber.
Can I access Mounjaro without going through my GP?
Yes. Private prescription routes do not require NHS referral — you can have an online consultation with a UK-licensed prescriber independently. However, NHS access does require a GP referral to specialist weight management services.
Is Mounjaro vs Wegovy better for upset stomach?
Both are GLP-1-based and cause similar digestive side effects initially. Wegovy (semaglutide) is a single GLP-1 agonist; Mounjaro is a dual GIP and GLP-1 agonist. Some people tolerate one better than the other — your prescriber can advise which might suit you if side effects are severe.
What should I eat if I have an upset stomach on Mounjaro?
Eat small, frequent meals; choose bland, low-fat foods like toast, rice, and boiled vegetables; avoid spicy, fatty, or greasy foods; and stay well hydrated with water and electrolyte drinks. Ginger tea may help nausea. If symptoms are severe, speak to your prescriber immediately.
Ready to take control of your weight loss journey?
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