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: Heartburn and Indigestion Comparison
Both Mounjaro and Wegovy can cause heartburn and indigestion as common side effects, but the mechanisms and individual experiences differ. If you're considering either medication for weight management, understanding how they compare on gastrointestinal side effects is important. This guide compares the two UK-licensed GLP-1 (and GIP/GLP-1) medications side-by-side, using clinical trial data and NHS context, to help you make an informed decision with your prescriber.
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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 and Heartburn Rates
Mounjaro (tirzepatide) — In the SURMOUNT-1 trial, indigestion (dyspepsia) occurred in approximately 10–15% of participants across all doses, with acid reflux (gastro-oesophageal reflux disease or GERD) also reported in common side effect data. The dual GIP and GLP-1 mechanism may contribute to these gastrointestinal effects.
Wegovy (semaglutide) — In the STEP-1 trial, indigestion was reported as a common side effect affecting up to 1 in 10 participants. As a GLP-1-only agonist, Wegovy's mechanism differs from Mounjaro's dual action, though gastrointestinal side effects remain frequent with both.
- Both medications slow gastric emptying (how quickly food leaves the stomach), which can trigger or worsen heartburn
- This slowing effect typically reduces over time as your body adjusts
- Individual variation is significant — some people experience mild symptoms, others more severe
- Use our GLP-1 weight loss calculator to explore potential outcomes at different doses, and discuss heartburn risk with your prescriber during your clinical assessment
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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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Suitability and Individual Factors
Neither Mounjaro nor Wegovy is absolutely contraindicated for people with existing heartburn or GERD — but your prescriber must assess your individual medical history. If you have severe or uncontrolled reflux, gastroparesis, or severe digestive conditions, one medication may be preferred over the other, or neither may be suitable.
Practical steps to manage heartburn on either medication:
- Take the medication with food if tolerated
- Avoid large meals — eat smaller, frequent meals instead
- Stay upright for 30 minutes after eating
- Avoid acidic, spicy, or fatty foods
- Discuss antacids or proton pump inhibitors (PPIs) with your GP — these can be used alongside either medication
- Report persistent or worsening heartburn to your prescriber — dose adjustment may help
Your prescriber will weigh heartburn risk against the potential weight loss benefits specific to your health situation.
- 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 worse heartburn than Wegovy?
Clinical trials don't show Mounjaro causes significantly worse heartburn than Wegovy — both report indigestion as a common side effect in 10–15% of users. Individual experiences vary widely. Your personal heartburn history matters more than the medication choice. Always discuss your medical history with your prescriber.
Can I take antacids while on Mounjaro or Wegovy?
Yes, you can use over-the-counter antacids or prescribed proton pump inhibitors (PPIs) alongside either medication if needed. However, GLP-1 medications may slow absorption of some oral medicines — inform your GP of all medications you're taking. Your prescriber can advise on timing.
Will heartburn from Mounjaro or Wegovy improve over time?
Yes, for many people. Gastrointestinal side effects often reduce as your body adjusts to the medication, typically within 2–4 weeks. The slowed gastric emptying that causes heartburn usually normalises over time. If heartburn persists beyond 8 weeks, discuss with your prescriber about dose adjustment.
Which medication is better if I already have acid reflux (GERD)?
Neither medication is inherently "better" for existing GERD — both can trigger or worsen reflux. Your prescriber will assess your individual GERD severity, current control, and medications before recommending either. Some people with mild GERD tolerate GLP-1 medications well with dietary adjustments; others need to explore alternatives.
Is heartburn a reason to stop taking Mounjaro or Wegovy?
Not automatically. Heartburn is common but manageable for many users. Your prescriber may suggest dietary changes, antacids, smaller meals, or dose adjustment before stopping. However, if heartburn is severe, uncontrolled, or affects your quality of life significantly, stopping is a valid option worth discussing with your GP.
Ready to take control of your weight loss journey?
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