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 Tummy Ache vs Wegovy — Which Causes More?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) can cause stomach discomfort, including nausea, bloating, and abdominal pain — particularly when starting or increasing doses. Tummy ache is a very common side effect of both weight loss injections in the UK. This guide compares how stomach symptoms differ between the two medications, what causes them, and how they typically progress over time. Understanding these differences can help you discuss options with your prescriber when deciding which prescription-only treatment might suit you.
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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 Stomach Side Effects Compare Between Mounjaro and Wegovy
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. Clinical trials showed that nausea, vomiting, and stomach pain affected more than 1 in 10 patients, making gastrointestinal side effects very common. The mechanism behind this is that tirzepatide slows gastric emptying — the rate at which food moves from your stomach to your small intestine — which can cause feelings of fullness, bloating, and discomfort.
Wegovy (semaglutide) is a single GLP-1 receptor agonist. In the STEP clinical trials, nausea and stomach symptoms were similarly common, affecting up to 1 in 10 people. However, because semaglutide uses only one hormone pathway (not dual action), some patients experience slightly different symptom patterns — though both medications slow gastric emptying.
- Both are prescription-only and require clinical assessment before use
- Stomach symptoms typically improve after 4–8 weeks as your body adjusts
- Starting at the lowest dose and titrating gradually (increasing slowly over weeks) reduces stomach side effects significantly
- Taking your weekly injection at the same time each week helps your body anticipate and adapt to the medication
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Why Tummy Ache Happens and When It Usually Improves
Stomach pain and nausea occur because both Mounjaro and Wegovy slow how quickly your stomach empties food into your intestines. This delayed gastric emptying makes you feel fuller faster and for longer — which is how these medications help with weight management — but it also causes temporary bloating, discomfort, and indigestion.
Good news: Most patients find that stomach symptoms reduce significantly within 4–8 weeks as your digestive system adapts to the medication. Starting at the lowest dose (2.5 mg for Mounjaro, 0.25 mg for Wegovy) and waiting at least 4 weeks before increasing to the next dose gives your body time to adjust. Eating smaller, lighter meals and staying hydrated also helps reduce tummy ache. If stomach symptoms persist or worsen, your prescriber may adjust your dose or suggest you switch medications — this is a decision made during your ongoing clinical assessment.
- 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 Mounjaro tummy ache worse than Wegovy?
Clinical trials show both cause similar rates of stomach discomfort. Individual experiences vary significantly. Some patients tolerate Mounjaro better, others prefer Wegovy. Your prescriber will assess which is more suitable for you based on your health history and how your body responds during dose titration.
How long does tummy ache last on these injections?
Most people experience reduced stomach symptoms within 4–8 weeks as their body adjusts. If symptoms persist beyond 8 weeks, inform your GP or prescriber. Slow dose escalation (waiting 4 weeks between increases) significantly reduces the severity of temporary stomach discomfort.
Can I reduce stomach pain by changing when I take the injection?
Taking your weekly injection at the same time each week helps your body anticipate the dose. Eating smaller, lighter meals and staying well hydrated can also reduce bloating and tummy ache. Avoid high-fat or heavy foods immediately after injection, which can worsen nausea and discomfort.
Should I stop the injection if I get bad stomach pain?
Do not stop without speaking to your prescriber first. Severe stomach pain spreading to your back could indicate pancreatitis and requires urgent medical attention — contact your GP or NHS 111 immediately. Mild to moderate discomfort usually improves with time and does not require stopping treatment.
Which GLP-1 injection should I choose to avoid tummy ache?
There is no assured way to avoid stomach side effects — both Mounjaro and Wegovy cause them in similar proportions. Your prescriber will assess your individual health to recommend the best option. Factors like kidney function, diabetes status, and previous medication tolerances influence this decision more than the specific symptom itself.
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