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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Mounjaro and Codeine: What You Need to Know
If you're taking Mounjaro (tirzepatide) and considering codeine pain relief, it's important to understand how these medicines interact. Mounjaro can slow gastric emptying and affect how other medications are absorbed, which may impact codeine's effectiveness or increase side effects. This guide explains the interaction, when to seek advice from your GP, and what to tell your prescriber about all medications you're taking before starting tirzepatide or any weight management treatment.
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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 and Codeine May Interact
Mounjaro (tirzepatide) is a once-weekly GIP and GLP-1 receptor agonist that slows how quickly food and medications move through your digestive system. Codeine is an opioid pain reliever commonly used for moderate pain relief in the UK. When taken together, several interaction concerns emerge:
- Reduced codeine absorption: Slower gastric emptying may delay or reduce how much codeine your body absorbs, potentially reducing its pain-relieving effect
- Increased side effects: Both medications can cause nausea, constipation, and dizziness — combining them may amplify these effects
- Opioid risks: GLP-1 medications may increase opioid-related side effects, particularly nausea and constipation
This interaction is not absolute — many people take both safely — but it requires informed prescriber oversight. Always inform your GP or prescriber that you take Mounjaro before using codeine, and inform them if you take regular codeine before starting tirzepatide treatment.
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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.
What Your Prescriber Needs to Know
Before starting Mounjaro or taking codeine while on tirzepatide, provide your prescriber with a complete medication list including all pain relief medications, over-the-counter painkillers, and any opioids. Your GP or specialist can assess the specific risk based on your health, dose of Mounjaro, and frequency of codeine use. Alternative pain relief options — such as paracetamol, ibuprofen (if appropriate), or non-opioid medications — may be recommended. If codeine use is necessary, your prescriber may adjust timing, dose, or monitoring. Never stop taking Mounjaro or change your pain medication without medical guidance.
- 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.
Can I take paracetamol or ibuprofen instead of codeine while on Mounjaro?
Yes. Paracetamol is typically safe with Mounjaro. Ibuprofen may also be used, but discuss with your GP, especially if you have kidney issues or take other medications. These are often preferred alternatives to opioids when taking GLP-1 weight loss medications.
Will Mounjaro reduce how well codeine works?
Possibly. Mounjaro slows gastric emptying, which may delay codeine absorption and reduce its effectiveness. Your prescriber can advise on timing between doses or suggest alternatives if pain relief is inadequate.
What should I tell my GP before starting Mounjaro?
Tell your GP about all medications you take, including pain relief, prescription medicines, over-the-counter products, and supplements. Mention if you regularly use codeine or other opioids. This helps your prescriber assess whether Mounjaro is suitable and what monitoring may be needed.
Are constipation and nausea worse if I take Mounjaro and codeine together?
Both medications can cause these side effects independently, so combining them may increase the risk. Your prescriber can discuss strategies to manage constipation and nausea, such as adjusting dose timing, increasing fluids, or using stool softeners if needed.
What if I need pain relief while waiting to start Mounjaro?
Discuss pain management options with your GP before starting tirzepatide. Your prescriber can recommend safe alternatives and confirm what's appropriate once you begin Mounjaro. Never self-manage by stopping either medication without medical advice.
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.