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 Slow Down Digestion?
Yes, Mounjaro (tirzepatide) can slow down how quickly food moves through your stomach — a process called gastric emptying. This effect is one of the ways it helps reduce appetite and support weight loss. However, this slowdown typically decreases over time as your body adjusts to the medication. Understanding how Mounjaro works, along with its cost and how to access it in the UK through NHS or private routes, can help you make informed decisions about weight management options.
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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.
How Mounjaro Affects Digestion and UK Private Prescription Costs
Mounjaro is a dual GIP and GLP-1 receptor agonist — it works by activating two natural hormone pathways that regulate appetite and blood sugar. One of its effects is slowing gastric emptying, which means food stays in your stomach longer, helping you feel fuller for longer. This contributes to reduced food intake and supports weight loss.
However, this digestive slowdown is typically a short-term effect. Most people find it decreases as their body adjusts to the medication over weeks to months. The slowing of digestion is not the same as gastroparesis (a serious digestive condition) — it's a managed, temporary effect.
UK prescription costs: Private Mounjaro prescriptions through online consultations cost approximately £180–£320 per month, depending on your dose. Starting doses (2.5 mg) are typically cheaper than maximum doses (15 mg). These are private costs; NHS prescriptions have no medication cost if accessed through specialist services, though NHS access is limited and varies by region (ICB). A tirzepatide course typically requires a clinical assessment before prescription to ensure eligibility.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
NHS vs Private Access: How to Get Mounjaro in the UK
NHS route: Mounjaro is available through NHS tier 3 specialist weight management services, but access is limited. You must be referred by your GP and meet specific eligibility criteria (BMI ≥30, or ≥27 with weight-related health conditions). Waiting lists can be months or years in many areas, and availability varies significantly by region.
Private route: Private online prescribing offers faster access. You complete an online consultation with a UK-licensed prescriber, who assesses your medical history and decides if Mounjaro is suitable. If approved, a prescription is issued to a GPhC-registered pharmacy. The process typically takes 2–5 working days. Private prescriptions require a Mounjaro KwikPen (the pre-filled injection device) to be dispatched to your address. Use our GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to check your current eligibility threshold.
- 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.
Does slowed digestion from Mounjaro cause constipation?
Slowed gastric emptying can contribute to constipation, which is a common side effect reported in clinical trials. However, many people also experience diarrhoea with Mounjaro. Side effects vary — keeping hydrated, eating fibre-rich foods, and staying active can help manage constipation if it occurs.
How long does the digestion-slowing effect last?
The most noticeable gastric emptying slowdown typically occurs in the first 4–8 weeks and tends to decrease as your body adjusts. By 12–16 weeks, many people find the effect is much less pronounced. Individual experiences vary significantly.
Can I take other medications if Mounjaro slows my digestion?
Mounjaro can reduce how quickly your stomach absorbs oral medications, potentially affecting their effectiveness. Inform your prescriber of all medications you take. If you use oral contraceptives, add a barrier method or switch contraception for 4 weeks after starting and after each dose increase.
What's the difference between Mounjaro and Wegovy for digestion?
Both slow gastric emptying, but Mounjaro (tirzepatide — a dual GIP and GLP-1 agonist) and Wegovy (semaglutide — GLP-1 only) work through different mechanisms. Mounjaro's dual action may produce different digestive effects for some users, though both can cause nausea, diarrhoea, and constipation.
Is there a cost difference between NHS and private Mounjaro prescriptions?
If you access Mounjaro through NHS specialist services, there's no medication cost once prescribed. Private prescriptions cost £180–£320 monthly depending on dose and provider. However, NHS waiting lists can be very long, and private access offers faster routes to treatment.
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.