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
Free tracker included with every download this week
Can You Take Mounjaro With Codeine?
If you're taking Mounjaro (tirzepatide) and need codeine for pain relief, you may be concerned about potential interactions between these two medicines. Mounjaro may slow how quickly food leaves your stomach, which can affect how your body absorbs other oral medications—including codeine. It's essential to inform your GP or prescriber about all medications you're taking before starting Mounjaro or adding any new medicines to your routine. This guide explains what you need to know about combining these medicines safely.
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Calculate Your BMI Instantly
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 Medication Absorption
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that works partly by slowing gastric emptying—the rate at which food and other substances move through your stomach. This delayed stomach emptying can reduce the absorption of oral medications taken at the same time. Codeine, an opioid painkiller, is taken by mouth and depends on normal stomach function for proper absorption into your bloodstream. When Mounjaro slows this process, codeine absorption may be delayed or reduced, potentially affecting its pain-relief effectiveness. However, individual responses vary significantly depending on your Mounjaro dose, how long you've been taking it, and other personal factors. Your prescriber needs to know about any pain relief needs so they can assess whether codeine is appropriate for you or suggest safer alternatives.
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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 GP or Prescriber Needs to Know
Before starting Mounjaro or if you already take it and need pain relief, inform your healthcare provider of all medications—including over-the-counter painkillers, prescription medicines, and herbal supplements. Your GP can advise whether codeine is safe in your circumstances or recommend alternatives like paracetamol or ibuprofen, which may be more suitable. Never adjust codeine dosing or stop taking either medicine without guidance. If you experience reduced pain relief while on Mounjaro, or unexpected side effects, contact your GP promptly. Some patients may need adjusted pain management strategies while using Mounjaro, so personalised medical advice is essential.
- 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.
Will codeine work less effectively if I'm taking Mounjaro?
Possibly. Mounjaro slows stomach emptying, which may delay or reduce codeine absorption. However, effects vary between individuals. Your GP can assess whether codeine is suitable for you or suggest alternatives like paracetamol, which isn't affected by Mounjaro in the same way.
Can I take codeine and Mounjaro together safely?
Only under guidance from your GP or prescriber. You must inform them of all medications before starting Mounjaro. They'll assess your individual situation and recommend whether codeine is appropriate or if a different painkiller would be safer and more effective.
What painkillers are safer to take with Mounjaro?
Paracetamol and ibuprofen are generally considered safer options because they're absorbed differently than codeine. However, individual circumstances vary. Always check with your GP before taking any new medication alongside Mounjaro to ensure it's right for you.
How long does Mounjaro affect medication absorption?
The stomach-emptying effect can persist while you're taking Mounjaro. It typically reduces over time as your body adjusts, but individual timelines vary. Your GP can advise on medication timing and absorption for your specific situation.
Should I stop codeine if I start taking Mounjaro?
Never stop any medicine without medical advice. Contact your GP if you're considering starting Mounjaro while taking codeine. They'll review your pain management options and adjust your treatment plan if needed to keep you safe and comfortable.
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.