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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Constipation on Mounjaro vs Wegovy — What's the Difference?
Constipation and digestive issues are common side effects of both Mounjaro (tirzepatide) and Wegovy (semaglutide) weight loss injections, but they don't affect everyone equally. Both medications slow how quickly food moves through your stomach and intestines, which can lead to constipation, diarrhoea, or both at different stages of treatment. This guide compares how these prescription weight loss medications affect digestion and what you can do to manage it. If you're experiencing severe or persistent constipation on either medication, always speak to your prescriber for personalised advice.
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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 Affect Digestion Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways that regulate appetite and digestion. Wegovy (semaglutide) is a GLP-1-only agonist. This difference in mechanism affects how they slow gastric emptying (the rate food leaves your stomach).
In the SURMOUNT-1 clinical trial, constipation was reported by approximately 20–25% of Mounjaro users across all doses. In the STEP-1 trial with Wegovy, constipation affected around 20% of participants. Both medications can cause constipation, but the timing and severity vary between individuals.
- Mounjaro: Dual mechanism may cause more variable digestive effects initially, with some users experiencing alternating constipation and diarrhoea as their body adjusts
- Wegovy: Single GLP-1 mechanism often produces more predictable digestive patterns, though constipation remains common during dose escalation
- Both: Constipation typically improves after the first 4–8 weeks as your body adapts to the medication
Important note: Individual experiences vary significantly. Some users have no constipation at all, while others find it the most bothersome side effect.
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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.
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Managing Constipation and Knowing When to Seek Help
Whether you're on Mounjaro or Wegovy, the same digestive management strategies usually help: increase water intake to at least 8–10 glasses daily, eat high-fibre foods (gradually, to avoid bloating), and stay physically active. Some people find that a daily fibre supplement or stool softener helps, but always check with your prescriber before adding new supplements, as some can interfere with medication absorption.
Seek urgent medical attention if you experience severe, persistent stomach pain radiating to your back (possible pancreatitis), or constipation lasting more than a week despite interventions. If constipation is mild but affecting your quality of life, ask your prescriber about dose timing or whether slowing your dose escalation might help. Use our free GLP-1 weight loss calculator to understand what dose might be right for you—and discuss any side effect concerns with your prescriber before or during dose increases.
- 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 on Mounjaro or Wegovy?
Both medications can cause constipation in roughly 20% of users. Mounjaro's dual mechanism may cause more variable digestive effects initially, while Wegovy's single GLP-1 action often produces more predictable patterns. Individual experiences vary significantly—some people have no constipation at all. Your prescriber can help determine which medication suits you best.
How long does constipation last on these injections?
Constipation typically improves after 4–8 weeks as your body adapts. If it persists beyond 8 weeks or worsens, speak to your prescriber. They may adjust your dose schedule, recommend dietary changes, or suggest other interventions tailored to your situation.
Can I take laxatives while on Mounjaro or Wegovy?
Over-the-counter stool softeners and fibre supplements are generally safe, but always inform your prescriber before using laxatives regularly. Some medications can affect how your body absorbs oral medicines, so your prescriber needs to know what you're taking alongside your injection.
What should I eat if I'm constipated on a GLP-1 injection?
Increase water intake, eat soluble fibre gradually (oats, vegetables, fruits), and stay active. Avoid high-fat foods if diarrhoea is also a problem. Start with small amounts of fibre to prevent bloating. Your GP or a registered dietitian can provide personalised dietary advice for your situation.
Does constipation mean the medication isn't working?
No. Constipation is a side effect of how the medication works (slowing stomach emptying), not a sign of effectiveness. Weight loss and constipation are independent outcomes. Many people experience both weight loss and temporary digestive changes. Track your progress with our free progress tracker to monitor weight changes separately from side effects.
Ready to take control of your weight loss journey?
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