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 — What You Need to Know
Dark stools can occur whilst taking Mounjaro (tirzepatide), though they are not among the most commonly reported side effects in UK clinical practice. This guide explains what dark stools may indicate, when to contact your GP, and how to access Mounjaro safely in the UK. If you're considering Mounjaro as part of your weight management journey, understanding potential side effects helps you make an informed decision alongside 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.
What Causes Dark Stools on Mounjaro?
Dark stools whilst taking Mounjaro may indicate gastrointestinal bleeding or changes in bowel function. Mounjaro slows how quickly food moves through your stomach, which can affect stool appearance and consistency. Dark or black stools (medically called melena) can occasionally signal bleeding in the upper digestive tract, though this is rare. Common gastrointestinal side effects of tirzepatide include constipation and diarrhoea — either can cause stool colour changes. Other possible causes include dietary changes (reduced food intake on Mounjaro may alter stool appearance) or increased stomach acid. The important distinction: if dark stools are accompanied by abdominal pain, vomiting, or dizziness, seek medical attention promptly. Occasional dark stools after dietary changes are usually harmless, but persistent dark stools warrant a GP consultation to rule out any underlying issue.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Accessing Mounjaro in the UK — NHS and Private Routes
NHS access: Mounjaro is available through specialist tier 3 weight management services, typically requiring a GP referral. NICE has approved tirzepatide for NHS use, though availability and waiting times vary significantly by region (Integrated Care Board). Private access: Mounjaro is available through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber. This route typically offers faster access without NHS waiting lists. Whether accessing via NHS or privately, your prescriber will assess your medical history, current medications, and eligibility before issuing a prescription. Tirzepatide costs vary by dose — the KwikPen device comes pre-loaded with four weekly injections. Understanding both routes helps you choose the access method that suits your timeline and circumstances.
- 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 common side effect of Mounjaro?
Dark stools are not listed among the very common or common side effects in clinical trials. The most frequent side effects are nausea, diarrhoea, and constipation. However, because Mounjaro affects digestion, stool appearance changes can occasionally occur. If you experience dark stools, mention them to your GP to ensure there is no underlying cause requiring attention.
When should I contact my GP about dark stools on Mounjaro?
Contact your GP immediately if dark stools are accompanied by severe abdominal pain, vomiting, dizziness, or signs of bleeding. If dark stools persist for more than a few days without other symptoms, inform your GP at your next opportunity. They can assess whether it is a harmless dietary effect or something requiring investigation. Never ignore symptoms that concern you.
Can I use the MyWeightCheck BMI calculator to track health changes on Mounjaro?
Yes. Our free BMI calculator helps you monitor your progress and understand whether your weight changes are moving you towards a healthier BMI range according to NHS guidelines. Tracking your BMI alongside any side effects you experience can help your GP assess how well Mounjaro is working for you and whether any adjustments are needed.
How much does Mounjaro cost on private prescription in the UK?
Private prescription costs for Mounjaro (tirzepatide) typically range from £130–£320 per month depending on your dose and the provider. Starting doses (2.5 mg) cost less than maximum doses (15 mg). Private online prescribers assess your eligibility and prescribe based on your individual health. Always check the specific cost with your chosen provider before starting treatment.
What is the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro uses two hormone pathways, which may explain slightly higher average weight loss in clinical trials. Both are prescription-only, available through NHS specialist services or private online assessment. Your GP or prescriber can discuss which may be more suitable based on your health profile.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.