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: Digestive Side Effects Compared
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections used in the UK, and both commonly cause digestive side effects including wind, bloating, and changes to bowel habits. These side effects are usually temporary and tend to improve as your body adjusts to the medication. Understanding how the two medications compare can help you and your GP decide which might be more suitable for you. Our free BMI calculator can help you check your eligibility before discussing weight management options with your healthcare provider.
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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.
How Mounjaro and Wegovy Cause Digestive Changes
Both medications work as GLP-1 (and in Mounjaro's case, dual GIP+GLP-1) receptor agonists, which slow how quickly food leaves your stomach. This delayed gastric emptying can lead to wind, bloating, constipation, or diarrhoea. Mounjaro uses a dual mechanism — activating both GIP and GLP-1 pathways — while Wegovy activates GLP-1 only. Clinical trial data from SURMOUNT (Mounjaro) and STEP (Wegovy) both report digestive side effects as very common, affecting more than 1 in 10 participants.
- Mounjaro: Nausea (29%), diarrhoea (23%), vomiting (8%), constipation (22%), abdominal pain (18%)
- Wegovy: Nausea (25%), diarrhoea (21%), vomiting (6%), constipation (19%), abdominal pain (17%)
The rates are similar between the two medications. Individual experience varies — some people experience mild symptoms for a few weeks, whilst others notice more persistent changes. Staying hydrated, eating smaller meals, and adding fibre gradually can help manage these effects.
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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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Which Medication Might Suit You Better?
The choice between Mounjaro and Wegovy is not primarily based on digestive side effects — both cause similar rates of wind and bloating. Your GP or prescriber will consider your overall health, other medications, dose requirements, and personal circumstances. Mounjaro requires fewer injections (once weekly at any dose), whilst Wegovy starts at a lower dose and escalates over 17 weeks. Try our free GLP-1 weight loss calculator to explore potential outcomes with different medications. Both require a UK prescription following clinical assessment, and both are typically prescribed through NHS specialist services or private online prescribing with appropriate medical oversight.
- 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.
Do Mounjaro and Wegovy cause different types of wind or bloating?
Both medications cause similar digestive changes because they both slow gastric emptying. The type and severity of wind, bloating, or constipation varies by individual, not significantly between the two medications. Clinical trials show comparable rates — around 18–23% report abdominal pain, and similar percentages report diarrhoea or constipation. Your prescriber can advise on managing these symptoms.
Which medication has fewer digestive side effects overall?
Clinical data shows Mounjaro and Wegovy have nearly identical rates of digestive side effects. Nausea affects approximately 25–29% of users with both medications. Neither has a clear advantage in reducing wind or bloating. Individual response varies widely — what matters most is how your body responds, which is why your prescriber will monitor you closely.
Do digestive side effects from Mounjaro and Wegovy go away?
Yes, in most people. Digestive side effects are typically most noticeable in the first 4–8 weeks and improve as your body adjusts. By 12 weeks, many people report significant improvement. However, some people experience ongoing mild changes like altered bowel habits. Your GP can offer strategies to manage persistent symptoms.
Can I switch from Mounjaro to Wegovy if digestive side effects are too much?
Possibly, but discuss this with your prescriber first. Stopping and switching medications requires careful medical oversight. Often, side effects improve with time rather than requiring a medication switch. Your prescriber may adjust your dose or recommend management strategies before considering an alternative medication. Never stop or switch without guidance.
Should digestive side effects influence my choice between Mounjaro and Wegovy?
Not as the primary factor. Since both medications cause similar digestive side effect rates, other factors matter more — your BMI, other health conditions, injection frequency preference (Mounjaro is once weekly from the start), and your prescriber's recommendation. Use our free BMI calculator to check your eligibility, then discuss all options with your GP.
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