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 Dark Stools vs Wegovy — What's the Difference?
Dark stools can occur with both Mounjaro and Wegovy, but understanding the difference between these two prescription weight management injections helps you know when to seek medical advice. Both are GLP-1 or GIP/GLP-1 receptor agonists licensed in the UK, but they work through slightly different mechanisms. This guide explains what dark stools mean on each medication, whether they're expected, and when to contact your GP. Individual responses vary significantly, so what one person experiences may differ from another.
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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 Mounjaro and Wegovy Differ in Mechanism and Side Effects
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways. This dual action may contribute to a slightly different side effect profile compared to Wegovy. Wegovy (semaglutide) activates GLP-1 only. Both medications slow gastric emptying — how quickly food leaves the stomach — which can affect stool appearance and consistency.
- Mounjaro clinical trials (SURMOUNT-1) showed average weight loss of 15–21% body weight
- Wegovy trials (STEP-1) showed average weight loss of approximately 15% body weight
- Both can cause changes in bowel habits including constipation and diarrhoea
- Dark or tarry stools are less commonly reported than nausea or diarrhoea but can occur with either medication
Dark stools may indicate slower transit time through the digestive system or, rarely, digestive bleeding. Always report persistent dark stools to your GP for assessment.
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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.
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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When to Seek Medical Advice and Individual Suitability
Dark stools that persist for more than a few days, are accompanied by stomach pain, or cause you concern warrant a GP appointment. Neither Mounjaro nor Wegovy should be assumed to cause dark stools automatically — this is not listed as a common side effect for either medication. Your prescriber will have assessed your individual health profile before issuing your prescription and can advise whether symptoms are expected or need investigation.
- Report any unusual stool changes to your GP promptly
- Track your symptoms using our free progress tracker to identify patterns
- Both medications require ongoing clinical assessment — inform your prescriber of all side effects
- Eligibility and suitability are determined individually through 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.
Are dark stools a normal side effect of Mounjaro or Wegovy?
Dark stools are not listed as common side effects of either medication. Common side effects include nausea, diarrhoea, and constipation. Dark or tarry stools can occasionally occur due to slower digestion, but persistent dark stools warrant GP assessment to rule out other causes. Always report unusual symptoms to your prescriber.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy is a GLP-1 agonist only. Both slow gastric emptying and reduce appetite, but the dual mechanism of Mounjaro may produce a slightly different side effect profile. Neither is superior — suitability depends on individual health assessment by your prescriber.
Can I compare expected side effects between Mounjaro and Wegovy?
Both medications share common side effects: nausea, diarrhoea, constipation, and stomach pain. Mounjaro clinical trials showed similar rates to Wegovy. Individual experience varies significantly. Use our GLP-1 weight loss calculator to explore potential outcomes, but discuss specific side effect concerns with your GP or prescriber.
Should I switch from one medication to the other if I experience dark sools?
Do not stop or switch medications without consulting your prescriber. Dark stools may have causes unrelated to your medication — your GP needs to assess this. Your prescriber can advise whether continuing, adjusting the dose, or switching is appropriate based on your individual circumstances.
How is eligibility determined for Mounjaro versus Wegovy in the UK?
Both require prescription and clinical assessment. Eligibility criteria are the same: BMI ≥30 or BMI 27–29.9 with weight-related health conditions. Your prescriber will assess which medication suits you based on your medical history, other medications, and individual suitability. Access is through NHS specialist services or private online prescribing.
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