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 Stomach Ache vs Wegovy: What's the Difference?
Stomach pain and abdominal discomfort are among the most common side effects reported with both Mounjaro (tirzepatide) and Wegovy (semaglutide) in the UK. Both medications are prescription-only GLP-1 receptor agonists approved for weight management, but they work through different mechanisms—Mounjaro is a dual GIP/GLP-1 agonist, while Wegovy is a GLP-1 agonist only. Understanding how their side effect profiles compare can help you make an informed decision alongside your prescriber during a clinical assessment.
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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.
How Clinical Trials Compared Stomach Symptoms
The SURMOUNT-1 trial for Mounjaro (tirzepatide) and the STEP-1 trial for Wegovy (semaglutide) both reported gastrointestinal side effects as very common. In SURMOUNT-1, stomach (abdominal) pain occurred in approximately 13–17% of participants across doses, alongside nausea and diarrhoea. STEP-1 reported similar frequencies of stomach pain. Importantly, most digestive symptoms were mild to moderate and tended to improve over time as the body adjusted to the medication. The timing and severity of stomach ache varied between individuals—some experienced discomfort within days of starting, others after dose increases. Neither trial showed that stomach pain was significantly more common with Mounjaro versus Wegovy, though individual responses differed widely. Symptom management strategies included eating smaller, lower-fat meals and staying hydrated, which UK healthcare providers often recommend during GLP-1 treatment.
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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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Which Medication Might Be Better for Your Stomach?
Because stomach ache frequency was comparable in both trials, choice between Mounjaro and Wegovy should not be based on stomach symptoms alone. However, Mounjaro's dual GIP/GLP-1 mechanism slows stomach emptying slightly differently than Wegovy's single GLP-1 pathway—this may affect individual tolerance. A prescriber will assess your personal health history, any existing digestive conditions, and other medications during a clinical assessment. If you have a history of gastroparesis or severe stomach problems, discuss this with your GP or prescriber, as they may recommend closer monitoring or an alternative approach. Once started on either medication, most people find digestive side effects manageable within 4–6 weeks. Use our free GLP-1 weight loss calculator above to explore potential outcomes at different doses, and track any symptoms with our progress tracker to share with your healthcare team.
- 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 ache more common with Mounjaro than Wegovy?
Clinical trial data (SURMOUNT and STEP) showed similar rates of stomach pain with both medications—approximately 13–17% of participants experienced abdominal discomfort. Individual experiences vary significantly. Neither medication is clearly associated with more stomach symptoms than the other based on available evidence.
Why do both Mounjaro and Wegovy cause stomach ache?
Both slow gastric emptying (how quickly food leaves the stomach), which can cause discomfort initially. Mounjaro's dual GIP/GLP-1 action and Wegovy's GLP-1 action both trigger this effect. Symptoms typically improve as your body adjusts, usually within 4–6 weeks.
What should I do if I get stomach pain on Mounjaro or Wegovy?
Contact your GP or prescriber if pain is severe or persistent. Manage mild discomfort by eating smaller, lower-fat meals, staying hydrated, and avoiding greasy foods. Paracetamol is generally safe, but check with your prescriber about other medications to ensure no interactions occur.
Can stomach pain mean I should switch from Mounjaro to Wegovy?
Not necessarily. Stomach symptoms are common with both medications and usually temporary. Your prescriber may suggest symptom management strategies, a slower dose increase, or staying on your current medication. Switching should only be done under medical guidance based on your overall response.
Does stomach ache get better over time on these medications?
Yes, for most people. Clinical trial participants reported that initial gastrointestinal discomfort improved significantly after 4–6 weeks as the body adjusted. If symptoms persist beyond 8 weeks, discuss this with your GP, as it may indicate your dose needs adjustment or a different approach is needed.
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