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 Fluoxetine?
If you're taking fluoxetine (an antidepressant) and considering Mounjaro (tirzepatide) for weight management, you may wonder whether it's safe to use both medications together. The short answer is that many people do take these medications concurrently, but it requires careful assessment by your prescriber. Fluoxetine and Mounjaro work through different mechanisms — one affects serotonin levels, the other is a GIP and GLP-1 receptor agonist — so direct pharmacological interactions are not typically expected. However, individual health circumstances, other medications, and how your body responds matter significantly. Always discuss your complete medication list with your GP or the prescriber assessing you for Mounjaro before starting or changing doses.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
Drug Interaction Considerations With Fluoxetine and Mounjaro
No major direct interaction has been identified between fluoxetine and Mounjaro in clinical guidance. Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) used to treat depression and anxiety, whilst Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that works through a completely different biological pathway. Because they target different systems, significant direct pharmacological interactions are unlikely.
However, several practical considerations apply: Mounjaro can slow gastric emptying (how quickly food leaves your stomach), which may theoretically affect how quickly fluoxetine is absorbed — though this effect typically diminishes over time as your body adjusts to the injection. If you're taking oral fluoxetine, inform your prescriber, as they may need to monitor your response more closely. Additionally, some people on SSRIs experience weight loss as a side effect, whilst others experience weight gain; starting Mounjaro introduces another variable that requires individualised assessment. Your prescriber will evaluate whether the combination is appropriate for your specific health picture.
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.
What to Tell Your Prescriber Before Starting Mounjaro
When seeking a Mounjaro prescription through your GP or a private online prescriber, always provide a complete list of all medications you're currently taking, including fluoxetine, the dose, and how long you've been taking it. This is essential during the clinical assessment. Mention any other health conditions, especially gastrointestinal issues, history of pancreatitis, or gallbladder problems, as Mounjaro carries specific contraindications. If you're registered as overweight or obese, use our free BMI calculator to confirm your current BMI category — this helps your prescriber assess your eligibility under UK weight management criteria. Your prescriber may ask about your mental health history, mood stability on fluoxetine, and any appetite or weight changes you've noticed since starting the antidepressant. This full picture helps them decide whether Mounjaro is suitable and at what starting dose.
- 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 Mounjaro stop fluoxetine from working?
No — Mounjaro does not interfere with fluoxetine's mechanism of action. Fluoxetine works on serotonin reuptake, whilst Mounjaro activates GIP and GLP-1 receptors. However, any medication changes can affect how you feel. Contact your GP immediately if you notice mood changes, increased anxiety, or worsening depression after starting Mounjaro.
Can fluoxetine and Mounjaro both cause nausea?
Yes — both medications can cause nausea, especially when first started or during dose increases. Fluoxetine nausea typically settles within 1–2 weeks; Mounjaro-related nausea may take several weeks. Your prescriber may adjust timing or recommend practical measures like eating smaller, frequent meals.
Do I need to tell my GP I'm taking fluoxetine before getting Mounjaro?
Absolutely — you must inform your prescriber of all medications, including fluoxetine. This is part of the clinical assessment required for Mounjaro prescription. There is no contraindication to using both, but your prescriber needs the complete picture to assess suitability and monitor you appropriately.
Can Mounjaro dose titration affect my fluoxetine levels?
Mounjaro's dose increases happen gradually (every 4 weeks minimum), following the standard titration schedule from the KwikPen. This slower approach allows your body to adjust. Fluoxetine levels are unlikely to be significantly affected, but any new symptoms warrant discussion with your GP.
What if I experience depression or mood changes after starting Mounjaro?
Contact your GP immediately. Whilst weight loss medications don't typically cause mood changes, some people report emotional shifts. Your GP can assess whether this relates to Mounjaro, your fluoxetine dose, or other factors, and adjust treatment as needed. Never stop fluoxetine suddenly.
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.