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 vs Wegovy: Stomach Pain and Digestive Side Effects
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) commonly cause stomach pain and digestive issues during the early weeks of treatment. Understanding how these two prescription-only weight management injections compare can help you make an informed decision with your prescriber. This guide compares the clinical trial data, mechanisms of action, and what UK patients typically experience with each medication.
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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.
Clinical Trial Data: Stomach Pain Frequency and Severity
Mounjaro (tirzepatide) in the SURMOUNT-1 trial showed that stomach pain and nausea were among the most common side effects, with gastrointestinal issues affecting significantly more participants than placebo. These symptoms typically emerged within the first 2–4 weeks and often improved with continued use.
Wegovy (semaglutide) in the STEP-1 trial similarly reported nausea and abdominal discomfort as very common side effects, with comparable frequency to Mounjaro in many cases. However, Mounjaro's dual mechanism (activating both GIP and GLP-1 pathways) may affect some patients differently than Wegovy's single GLP-1 pathway.
- Mounjaro: stomach pain reported in clinical trials at similar rates to Wegovy
- Wegovy: nausea and vomiting very common in early weeks, often settling after dose stabilisation
- Individual variation is significant—some patients tolerate one better than the other
- Dose escalation timing can influence symptom severity for both medications
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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.
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Which Medication Might Suit You Better?
The choice between Mounjaro and Wegovy depends on your individual health profile, not simply which causes "less" stomach pain—results vary considerably between patients. A licensed prescriber will assess you individually before recommending either medication, considering your medical history, current medications, and specific risk factors.
Both medications require a UK prescription and clinical assessment. Some patients find symptoms settle quickly with one medication; others may need to switch. Starting at the lowest dose and escalating gradually is standard practice with both—using our GLP-1 weight loss calculator can help you understand potential timelines.
If you experience severe, persistent stomach pain (especially pain radiating to your back), seek medical attention immediately, as this may indicate pancreatitis.
- 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.
Does Mounjaro cause more stomach pain than Wegovy?
Clinical trials show similar rates of gastrointestinal side effects between the two medications. However, individual responses vary significantly. Some patients tolerate one better than the other due to differences in their gut chemistry and how they respond to the GIP vs GLP-1 pathways. Your prescriber will assess which is most suitable for you based on your personal health profile.
How long does stomach pain last on Mounjaro or Wegovy?
Stomach pain and nausea typically emerge within the first 1–2 weeks of starting either medication and often improve significantly after 3–4 weeks as your body adjusts. However, some patients experience ongoing mild nausea during dose escalation. Taking these injections with food and staying hydrated may help. If symptoms persist or worsen, contact your prescriber.
Can I switch from Mounjaro to Wegovy if I have stomach pain?
Yes, switching between medications is possible if you experience intolerable side effects. However, this requires your prescriber's approval and guidance. They will determine the appropriate timing and dosing for any switch. Never stop either medication abruptly without medical advice, as this can cause rebound effects.
Are there ways to reduce stomach pain while using these injections?
Practical steps include eating smaller, blander meals; staying hydrated; avoiding fatty or spicy foods; taking injections consistently at the same time weekly; and moving gently after eating. Some patients find ginger, peppermint tea, or over-the-counter antacids helpful—discuss these with your prescriber first, as some medications may interact with GLP-1 treatments.
Should I choose Mounjaro or Wegovy based on stomach pain risk alone?
No—your prescriber will consider your overall health, BMI, any weight-related conditions, current medications, and medical history before recommending either treatment. Stomach pain risk alone is not the primary factor. Both medications are prescription-only and require individual clinical assessment to determine suitability for your specific circumstances.
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