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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Does Mounjaro Make You Constipated?
Yes, constipation is a common side effect of Mounjaro (tirzepatide), though not everyone experiences it. Mounjaro can slow how quickly food moves through your digestive system, which may cause constipation, particularly during the early weeks of treatment. Wegovy (semaglutide), another GLP-1 weight loss injection available in the UK, also carries constipation risk. However, the frequency and severity can differ between individuals due to different mechanisms and doses. Understanding what to expect helps you manage side effects effectively with your prescriber.
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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.
Mounjaro Constipation vs Wegovy — Clinical Evidence
Both Mounjaro and Wegovy can cause constipation, but clinical trial data shows slightly different patterns. In the SURMOUNT-1 trial (Mounjaro), constipation affected approximately 23% of participants at the 15 mg dose, making it one of the most common digestive side effects. In the STEP-1 trial (Wegovy 2.4 mg), constipation occurred in around 19% of participants.
The key difference lies in their mechanisms: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates only the GLP-1 pathway. This dual action in Mounjaro may affect gastric (stomach) motility differently than Wegovy alone. Both medications slow gastric emptying — how quickly food leaves your stomach — which helps with appetite control but can contribute to constipation, especially when combined with reduced food intake.
- Constipation typically occurs within the first 4–8 weeks of starting treatment
- Risk increases with higher doses (higher at 15 mg Mounjaro vs 2.5–5 mg)
- Severity often improves as your body adjusts over weeks 8–12
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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.
Managing Constipation — Practical Steps
If you experience constipation on Mounjaro or Wegovy, several evidence-based strategies can help. First, ensure adequate hydration — aim for at least 2 litres of water daily, as slowed digestion requires extra fluid. Increase soluble fibre gradually through vegetables, fruits, and oats, but avoid sudden large increases which can worsen bloating. Gentle movement and light exercise, even 10–15 minutes of walking daily, support digestive function. If constipation persists beyond 2 weeks or becomes severe, contact your prescriber — they may adjust your dose timing, recommend an osmotic laxative (like lactulose), or temporarily reduce your dose. Never self-medicate with harsh laxatives without guidance. Keep detailed notes using our free progress tracker to log both weight loss and digestive symptoms, helping your prescriber tailor your treatment effectively.
- 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 constipation worse with Mounjaro than Wegovy?
Clinical trials show constipation rates are similar (approximately 23% for Mounjaro 15 mg vs 19% for Wegovy 2.4 mg). Individual experiences vary significantly. Some people tolerate one medication better than the other due to differences in their gut sensitivity and dose response. Your prescriber can assess which is more suitable based on your medical history and any previous GLP-1 experiences.
How long does constipation last when starting Mounjaro?
Constipation typically peaks in weeks 2–4 after starting or increasing your Mounjaro dose, then improves by weeks 8–12 as your body adjusts. However, some people experience ongoing mild constipation throughout treatment. Staying hydrated and increasing fibre gradually helps. If it persists beyond 12 weeks, inform your GP or prescriber — dose adjustment may be needed.
Can I prevent constipation before starting Mounjaro?
You cannot prevent it entirely, but preparation helps. Before starting your tirzepatide KwikPen, increase your daily water intake gradually, add more soluble fibre to your diet (vegetables, oats), and plan regular light exercise. These changes reduce the shock to your digestive system when you begin treatment. Avoid high-fat foods, which can worsen constipation when combined with slowed gastric emptying.
Are there any constipation medicines safe to use with Mounjaro?
Osmotic laxatives like lactulose or polyethylene glycol (PEG) are generally safe alongside Mounjaro, but always inform your prescriber before using any laxative. Avoid stimulant laxatives (senna, bisacodyl) without guidance, as they can cause dependency with prolonged use. Your prescriber may recommend specific products that work well with GLP-1 medications and reduced food intake.
Does Mounjaro dose affect constipation risk?
Yes — constipation risk increases with higher doses. The 2.5 mg starting dose causes constipation in fewer people than the 15 mg maximum dose. Your prescriber uses a gradual titration schedule (increasing every 4 weeks) partly to allow your digestive system to adjust. If constipation becomes problematic at a higher dose, dose reduction or staying at a lower effective dose may be appropriate.
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.