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 Cause More Constipation Than Wegovy?
Constipation is a common side effect of both Mounjaro and Wegovy, but the frequency and severity differ between these two weight loss injections. Mounjaro (tirzepatide) and Wegovy (semaglutide) work through similar but distinct mechanisms, which can affect how constipation develops during treatment. Understanding these differences helps you and your GP or prescriber make an informed decision about which prescription medication may be more suitable for your individual health profile and lifestyle.
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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.
Clinical Trial Data: Constipation Rates Compared
In the SURMOUNT-1 clinical trial for Mounjaro, constipation affected approximately 20–25% of participants at the highest dose (15 mg). The STEP-1 trial for Wegovy reported constipation in roughly 20% of participants on the maximum dose (2.4 mg). However, these rates vary significantly by dose — constipation is more common at higher maintenance doses than at lower starting doses.
The key mechanistic difference is important: Mounjaro is a dual GIP and GLP-1 receptor agonist, whereas Wegovy activates only the GLP-1 pathway. This dual action in Mounjaro may contribute to slightly different gastrointestinal effects, though constipation remains common with both medications. Importantly, constipation tends to be most bothersome in the first 4–8 weeks and often improves as your body adjusts to the medication. Individual responses vary considerably — some users experience minimal digestive changes, while others require active management.
- Mounjaro: constipation reported in ~20–25% at 15 mg dose
- Wegovy: constipation reported in ~20% at 2.4 mg dose
- Severity typically decreases after 8–12 weeks of consistent dosing
- Hydration, fibre intake, and movement all support management
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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.
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Managing Constipation and Choosing Between Them
If you have a personal or family history of constipation or digestive sensitivity, your prescriber should know this before starting either medication. Neither Mounjaro nor Wegovy is contraindicated in people with a history of constipation, but your prescriber may recommend starting at the lowest dose and escalating more slowly, or suggest preventative measures like increased fibre and hydration from day one.
Both medications require a prescription and clinical assessment — your GP or a licensed prescriber will evaluate your medical history, current medications, and lifestyle to determine which is more appropriate for you. Constipation alone is rarely a reason to exclude either option; instead, it's managed as part of your overall treatment plan. Many UK weight loss clinics now recommend proactive constipation strategies (fibre, hydration, activity) alongside medication initiation. Use our free GLP-1 weight loss calculator to explore potential outcomes, and discuss specific side effect concerns with your prescriber.
- 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 trial data shows constipation rates are similar — approximately 20–25% with Mounjaro and 20% with Wegovy at maximum doses. Severity varies by individual. The dual mechanism in Mounjaro (GIP + GLP-1) may cause slightly different digestive effects, but constipation is a known side effect of both medications. Your prescriber can help manage it with dietary changes and activity.
How long does Mounjaro constipation last?
Constipation is usually most noticeable in the first 4–8 weeks as your body adjusts to the medication. Many users report improvement by week 8–12 as tolerance develops. Staying hydrated, increasing fibre gradually, and maintaining physical activity significantly reduce duration and severity. If constipation persists beyond 12 weeks, discuss with your GP.
Can I take laxatives while on Mounjaro or Wegovy?
Yes, over-the-counter laxatives can be used safely alongside these medications. However, start with dietary approaches first — increased water, fibre-rich foods, and movement. If needed, gentle laxatives like docusate or osmotic options are generally safe. Always inform your prescriber about any medications or supplements you're taking.
Which injection should I choose if I'm prone to constipation?
Both carry similar constipation risk, so choice depends on other factors: Mounjaro's dual mechanism may suit some individuals, while Wegovy's single GLP-1 action may suit others. Your prescriber will assess your full medical history, including digestive health, to recommend the most suitable option. Proactive management strategies work well for both.
Is constipation a reason to stop Mounjaro or Wegovy?
Constipation alone is rarely severe enough to warrant stopping either medication. Most cases improve with management strategies — hydration, fibre, activity, and sometimes gentle laxatives. If constipation is severe or unmanageable despite these measures, speak to your GP or prescriber about dose adjustment or switching medications. ```
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