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: Drug Interactions and Safety
If you're taking Mounjaro (tirzepatide) and considering codeine, or already using both medications, it's important to understand potential interactions. Codeine is an opioid pain reliever commonly prescribed in the UK for moderate pain, whilst Mounjaro is a prescription-only GIP and GLP-1 receptor agonist used for weight management. Both affect how your body processes food and pain, so combining them requires careful consideration and GP guidance. This guide explains what you need to know about using these medications together safely.
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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 Work in Your Body
Mounjaro works by activating GIP and GLP-1 receptors, which regulate appetite, slow gastric emptying (how quickly food leaves your stomach), and support blood sugar control. Codeine is an opioid that binds to opioid receptors in the central and peripheral nervous system to relieve pain.
When combined, these medications can interact in several ways. Gastrointestinal effects: Both Mounjaro and codeine can slow digestion and cause constipation or nausea—common side effects of both drugs. Using them together may increase the likelihood and severity of stomach-related side effects. Opioid sensitivity: GLP-1 medications like semaglutide (Wegovy) have been shown to enhance opioid sensitivity in some patients, meaning you may feel the effects of codeine more strongly. Whilst Mounjaro works via a slightly different mechanism (dual GIP+GLP-1), caution is still advised.
Your GP or prescriber should assess your individual risk before recommending this combination. If you need pain relief whilst taking Mounjaro, alternatives like paracetamol or ibuprofen may be safer choices.
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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.
When You Might Need Both—And What to Do
There are situations where you may legitimately need both medications. For example, if you have chronic pain managed with codeine and are also using Mounjaro for weight management, stopping either medication abruptly isn't advisable. If this applies to you: Inform your prescriber of all medications you're taking—including over-the-counter painkillers. Your GP can review your pain management options and may suggest alternatives such as paracetamol, topical pain relief, or non-opioid medications that interact less with Mounjaro.
Never adjust doses or stop medications without professional guidance. Use our free GLP-1 weight loss calculator to track your progress independently, and keep your GP updated on any side effects or concerns. If constipation, severe nausea, or unusual pain sensation occurs, contact your GP immediately—these may indicate a problematic interaction.
- 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 codeine whilst taking Mounjaro?
Potentially, but only under GP supervision. Both medications can cause constipation and nausea. Your prescriber needs to assess your individual health and may recommend alternative painkillers like paracetamol or ibuprofen instead. Always inform your GP of all medications you're taking.
What are the main interactions between Mounjaro and codeine?
The primary concern is gastrointestinal side effects—both slow digestion and can cause constipation or nausea. GLP-1 medications may also enhance opioid sensitivity. Combined use increases the risk of stomach pain, constipation, and nausea severity. Individual risk varies.
What painkiller should I use instead of codeine with Mounjaro?
Paracetamol and ibuprofen (NSAIDs) are generally safer options alongside Mounjaro. They don't significantly interact with tirzepatide. However, always check with your GP or pharmacist before starting any new medication, as individual circumstances vary.
Is Mounjaro different from Wegovy in terms of opioid interactions?
Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist, whilst Wegovy (semaglutide) is a GLP-1-only medication. Evidence of opioid sensitivity is stronger for GLP-1 alone, but caution is advised with both. Your prescriber can advise on the safest option for your situation.
Should I stop my codeine prescription when starting Mounjaro?
No—never stop medication without professional guidance. Contact your GP first. They may adjust your dosing, suggest alternative painkillers, or monitor you more closely if both medications are medically necessary. Sudden medication changes can be unsafe.
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.