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 Duloxetine — What UK Patients Should Know
Mounjaro (tirzepatide) is a prescription-only weight management injection, whilst duloxetine is an antidepressant medication. Some patients take both together. This guide explains how these medications work, what research shows about combining them, and why clinical assessment is essential before starting either medication. Always discuss any medication combination with your GP or prescriber to ensure safety and suitability for your individual health.
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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 These Medications Work And Their Potential Interaction
Mounjaro is a dual GIP and GLP-1 receptor agonist — a tirzepatide injection that works by activating two natural hormone pathways to help regulate appetite, reduce food cravings, and support feeling fuller after meals. It's given once weekly as a Mounjaro KwikPen injection.
Duloxetine is a serotonin-noradrenaline reuptake inhibitor (SNRI) antidepressant used to treat depression, anxiety, chronic pain, and fibromyalgia. It works by affecting neurotransmitters in the brain.
Both medications can cause nausea and appetite changes. When taken together, these effects may be more pronounced. Research on this specific combination is limited, so close monitoring by your prescriber is essential. Neither medication directly counteracts the other, but your GP needs to assess whether combined use is appropriate for your individual circumstances, considering your medical history, other medications, and current health status.
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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.
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Clinical Considerations And What You Should Do
If you're already taking duloxetine and considering Mounjaro, or vice versa, your prescriber must be aware of both medications during your clinical assessment. Tell your GP or the prescriber conducting your online assessment about all medications, supplements, and health conditions you have.
Your prescriber will assess whether the combination is suitable for you, considering factors like your mental health stability, digestive health, and overall medication interactions. If approved, your GP or prescriber will monitor you more closely when starting Mounjaro to watch for worsening nausea or mood changes. You can use our free GLP-1 weight loss calculator to explore potential weight loss projections with Mounjaro at different doses, and our BMI calculator to track your baseline health metrics.
- 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 Mounjaro if I'm already on duloxetine?
Possibly, but only with prescriber approval. Tell your GP or online prescriber that you take duloxetine before starting Mounjaro. There's no direct contraindication, but your prescriber needs to assess whether the combination is safe for you personally, considering your mental health, digestive system, and other medications. Close monitoring may be needed.
Will Mounjaro interact with duloxetine?
There are no major direct drug-drug interactions between Mounjaro and duloxetine reported in clinical literature. However, both can cause nausea and appetite changes, so these effects may be more noticeable when combined. Your prescriber will monitor this carefully to ensure safety.
What should I tell my GP before starting either medication?
Always disclose: all current medications (including duloxetine), any history of depression or anxiety, digestive or stomach problems, pancreatitis history, gallbladder issues, kidney or liver disease, and any allergies. Honest disclosure helps your GP assess suitability and avoid unsafe combinations.
Can duloxetine affect how Mounjaro works?
Duloxetine doesn't directly reduce Mounjaro's weight loss effect. However, duloxetine can affect appetite and mood independently, so the combined effect on your eating patterns and weight may vary. Your prescriber considers this during assessment and monitoring.
What if I experience worse nausea or mood changes after starting Mounjaro?
Contact your GP immediately. Don't adjust either medication yourself. Your prescriber may need to slow your Mounjaro dose escalation, adjust duloxetine timing, or explore alternative options. Regular check-ins help catch problems early and keep you safe.
Ready to take control of your weight loss journey?
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