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 is a common side effect reported by people taking Mounjaro (tirzepatide), and it's also possible with Wegovy (semaglutide). Both are prescription-only weight management injections licensed in the UK, but they work slightly differently — which can affect how your digestive system responds. This guide explains the key differences between these medications, what the clinical trials showed about constipation rates, and how your GP or prescriber can help manage this side effect if it occurs during your treatment.
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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.
Mounjaro vs Wegovy — Clinical Evidence on Constipation
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two separate hormone pathways. In the SURMOUNT clinical trials, constipation was reported in approximately 20–24% of participants. Wegovy (semaglutide) is a GLP-1 receptor agonist only — activating a single pathway. In the STEP trials, constipation occurred in around 19–20% of participants at the highest dose (2.4 mg).
The dual mechanism of Mounjaro may affect gut motility differently than Wegovy's single mechanism, though individual responses vary significantly. Both medications slow how quickly food leaves your stomach (gastric emptying), which can contribute to constipation, especially during dose escalation when your body is adjusting to the medication.
- Mounjaro doses: 2.5 mg to 15 mg weekly
- Wegovy doses: 0.25 mg to 2.4 mg weekly
- Both require a UK prescription and clinical assessment
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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 — Prescriber Support and Suitability
If you experience constipation on either medication, your GP or prescriber can offer practical support. Increasing water intake, adding fibre-rich foods gradually, and gentle physical activity often help. Some people find constipation improves as their body adjusts after a few weeks at each dose level. If constipation persists or becomes uncomfortable, contact your prescriber — they may adjust your dose, suggest over-the-counter remedies, or discuss whether a switch to the alternative medication might suit you better.
Which medication is right for you depends on your individual health profile, medical history, and how your body responds. Neither Mounjaro nor Wegovy is universally "better" — suitability is determined through a clinical assessment by your prescriber. Use our GLP-1 weight loss calculator to explore projected outcomes, and discuss any digestive concerns with your healthcare provider before starting or adjusting any medication.
- 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 more common on Mounjaro or Wegovy?
Clinical trial data shows both medications have similar constipation rates (around 19–24%). Individual responses vary significantly. Mounjaro's dual mechanism may affect digestion differently than Wegovy's single mechanism, but this doesn't assures one causes more constipation than the other. Your personal experience depends on many factors including dose level, diet, hydration, and individual biology.
What should I do if I'm constipated on Mounjaro?
First, increase water intake, add fibre gradually (via vegetables, whole grains), and move more daily if possible. Constipation often improves within a few weeks as your body adjusts. If it persists, contact your prescriber — they may recommend a stool softener, adjust your dose, or discuss other options. Never stop the medication without medical guidance.
Can I switch from Mounjaro to Wegovy if I'm constipated?
A prescriber can discuss switching medications, though it's not assured the alternative will cause fewer digestive side effects. Any switch requires clinical assessment and prescription. Your GP can advise whether switching is appropriate for your health profile and whether waiting through the adjustment period is reasonable first.
How long does constipation last on these medications?
For many people, constipation improves within 2–4 weeks as the body adjusts to the medication. However, some experience it throughout treatment. If constipation is severe or doesn't improve, inform your prescriber promptly — there are management strategies available and they can assess whether continuing is appropriate.
Are there other digestive side effects I should know about?
Both Mounjaro and Wegovy commonly cause nausea, diarrhoea, and stomach discomfort, especially during dose escalation. These usually settle over time. Very common side effects occur in more than 1 in 10 people. If you experience severe, persistent stomach pain, contact your GP immediately — this could indicate pancreatitis and requires urgent assessment.
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