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
Does Fluoxetine Cause Weight Loss?
Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) commonly prescribed in the UK to treat depression, anxiety, and other mental health conditions. Weight changes on fluoxetine can go either way — some people experience weight loss, whilst others gain weight or experience no change at all. Individual responses vary significantly. If you're concerned about weight changes while taking fluoxetine, it's important to discuss this with your GP rather than stopping the medication without medical guidance. This guide explains how fluoxetine affects weight and what evidence-based options are available for weight management in the UK.
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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 Fluoxetine Affects Weight
Fluoxetine works by increasing serotonin levels in the brain, which can influence appetite and mood. Weight changes on fluoxetine are unpredictable and highly individual. Approximately 10–15% of people experience weight loss, whilst 10–15% experience weight gain. Many people notice no significant change at all. Initial weight loss sometimes occurs in the first few weeks due to reduced appetite or nausea as a side effect, but this often stabilises as your body adjusts to the medication. Some people experience increased appetite and weight gain later on. Weight changes can also be influenced by improved mood and activity levels — as depression improves, some people become more active, which may support weight loss, whilst others may have increased energy for eating.
- Weight loss in early weeks may reflect nausea or appetite suppression
- Weight gain can occur if appetite increases over time
- Mental health improvement may indirectly affect eating patterns and activity
- Genetic factors and individual metabolism play a significant role
Clinically BackedWeight Loss
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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.
Weight Management Options if Concerned About Weight on Fluoxetine
If you're experiencing weight gain or loss concerns on fluoxetine, speak to your GP before making any changes. Do not stop taking fluoxetine without medical advice, as this can trigger withdrawal symptoms and relapse of your mental health condition. Your GP can discuss several options: switching to a different antidepressant (some SSRIs are weight-neutral), adjusting your dose, or combining your medication with lifestyle support like dietitian referral or NHS weight loss programmes. For those with a BMI of 30 or above, or 27 or above with weight-related health conditions, prescription weight loss medication may be an option to discuss with your GP. The NHS offers structured weight management services — your GP can refer you to your local tier 3 weight management service if appropriate. You can also track your progress using our free progress tracker to monitor changes over time.
- 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 fluoxetine alone cause weight loss?
Fluoxetine may cause weight loss in some people, particularly in the first few weeks due to reduced appetite or nausea. However, weight changes vary widely between individuals — some experience weight gain, and many experience no significant change. Weight loss is not assured and depends on individual factors including metabolism, dose, how long you've been taking it, and changes in mental health or activity levels.
Is weight loss a common side effect of fluoxetine?
Weight loss is not listed as a common side effect of fluoxetine in UK prescribing information, though it does occur in some people. More commonly reported are either weight gain or no change. If you notice significant weight changes on fluoxetine, discuss this with your GP — they can assess whether it's related to the medication or other factors, and advise whether any adjustment is needed.
Should I stop taking fluoxetine if I'm gaining weight?
No — never stop fluoxetine without discussing it with your GP first. Stopping suddenly can cause withdrawal symptoms and mental health relapse. Weight gain on fluoxetine is usually manageable through lifestyle changes or switching to a different antidepressant if appropriate. Your GP can help you explore safe alternatives without risking your mental health.
What weight loss options are available alongside antidepressants?
If you're struggling with weight whilst on fluoxetine, your GP can refer you to an NHS weight management service, a dietitian, or a structured weight loss programme. For those eligible (typically BMI ≥30 or ≥27 with weight-related health conditions), prescription weight loss medication may be considered alongside your mental health treatment — this requires individual assessment and prescription.
Can I use weight loss injections if I'm taking fluoxetine?
Some people take GLP-1 weight loss medications (such as semaglutide in Wegovy) alongside antidepressants, but this requires careful assessment by a prescriber. These medications work differently from fluoxetine and are not alternatives to mental health treatment. Any combination of medications must be reviewed by your GP or a prescriber to ensure safety and check for interactions. Try our GLP-1 weight loss calculator to explore potential outcomes, but always discuss medication combinations with a qualified healthcare professional first.
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.