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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Tummy Ache On Mounjaro: What You Need To Know
Stomach pain and discomfort are among the most common side effects reported by people taking Mounjaro (tirzepatide) in the UK. If you're experiencing a tummy ache while on this prescription weight management injection, you're not alone — clinical data shows nausea and abdominal discomfort affect many users, especially when starting or increasing doses. This guide explains why it happens, how to manage it, and what your options are for accessing Mounjaro safely through the NHS or privately.
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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.
Why Does Mounjaro Cause Tummy Ache?
Mounjaro works as a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways that regulate appetite and digestion. One of its effects is slowing gastric emptying — the rate at which food leaves your stomach. This is why stomach discomfort, bloating, and nausea occur, particularly in the first few weeks or when your dose increases.
- Very common side effects (affecting more than 1 in 10 people): nausea, stomach pain, diarrhoea, and constipation
- Side effects often improve over time as your body adjusts — usually within 2–4 weeks at each dose level
- Gastrointestinal symptoms are more pronounced during dose titration (when you're moving from 2.5mg to 5mg to 10mg etc.)
- Food choices matter — fatty or heavy foods worsen symptoms; small, frequent meals help
When comparing Mounjaro vs Wegovy, both cause similar GI side effects, though the dual mechanism of tirzepatide may produce slightly different symptom patterns in some users.
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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: Cost & Routes
In the UK, you can access Mounjaro through two main routes: the NHS specialist services or privately. NHS route: Referral to a tier 3 weight management service from your GP; access varies significantly by region and waiting times can be lengthy. Private route: Online consultation with a UK-licensed prescriber, prescription issued if clinically suitable, dispensed by a GPhC-registered pharmacy. Private costs range from approximately £129–£250+ per month depending on your dose level (the KwikPen contains 4 weekly doses). Both routes require a clinical assessment to confirm eligibility and suitability. Our GLP-1 weight loss calculator can help you estimate potential outcomes at different doses before consulting a prescriber.
- 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 stomach pain on Mounjaro serious?
Mild to moderate stomach discomfort, nausea, and bloating are expected and usually improve within weeks. However, seek urgent medical attention if you experience severe, persistent stomach pain spreading to your back — this could indicate pancreatitis. Always contact your GP if symptoms are severe or prolonged.
When should tummy ache on Mounjaro improve?
Most users find GI symptoms settle within 2–4 weeks of starting each dose. Your body gradually adapts to the medication. If symptoms persist beyond 4 weeks or worsen significantly, speak to your prescriber about whether dose adjustment or temporary pausing might help.
Can I get Mounjaro on the NHS for weight loss?
Yes — NICE has approved tirzepatide (Mounjaro) for NHS use within specialist tier 3/4 weight management services. Access requires a GP referral and varies by region. Many areas have long waiting lists. Private online prescription offers faster access but costs approximately £129–£250+ monthly.
What's the difference between Mounjaro and Wegovy for side effects?
Both are GLP-1-type medications causing similar GI symptoms. Mounjaro is a dual GIP/GLP-1 agonist; Wegovy is GLP-1 only. Individual symptom patterns vary — some tolerate one better than the other. Your prescriber can advise which suits you based on your medical history.
How do I access a private Mounjaro prescription in the UK?
Complete an online health assessment with a UK-licensed prescriber, answer questions about your eligibility (BMI, health conditions), and if approved, receive a prescription to a GPhC-registered pharmacy. The entire process typically takes 1–3 days. Costs depend on dose — starting doses cost less than higher maintenance doses.
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.