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 Fluoxetine Cause Weight Loss?
Fluoxetine is an SSRI antidepressant that can affect weight in different ways depending on the individual. While some people experience weight loss initially, others gain weight over time. Weight changes from fluoxetine are not a reliable weight loss method. If you're struggling with your weight and looking for effective support, the UK has several prescription weight management options available for eligible adults. This guide explains how fluoxetine affects weight and outlines the evidence-based alternatives your GP can discuss with you.
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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 Fluoxetine Affects Weight
Fluoxetine can cause weight loss or weight gain — individual responses vary significantly. In the first few weeks, some people experience reduced appetite and lose weight. However, this effect often reverses over time as the body adapts. Many people taking fluoxetine long-term actually gain weight, particularly if the medication improves mood and appetite returns to normal.
- Initial phase (weeks 1–4): Some people experience appetite suppression and weight loss
- Adaptation phase (months 2–6): Weight loss often plateaus or reverses as the body adjusts
- Long-term: Weight gain is common, though not universal
Fluoxetine is an antidepressant, not a weight loss medication. It works by increasing serotonin levels to improve mood and anxiety — weight changes are a potential side effect, not the intended effect. If weight management is your primary goal, prescription weight loss medications approved by the NHS for weight management are a more appropriate option and have stronger evidence for sustained weight 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.
Prescription Weight Loss Options in the UK
If you have a BMI of 30 or above (or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure), you may be eligible for prescription weight loss treatment in the UK. Three medications are licensed for weight management:
- Mounjaro (tirzepatide) — Dual GIP/GLP-1 receptor agonist; clinical trials showed average weight loss of 15–21% of body weight
- Wegovy (semaglutide) — GLP-1 receptor agonist; average weight loss around 15% of body weight
- Saxenda (liraglutide) — Daily GLP-1 injection; evidence supports modest weight loss alongside lifestyle changes
These medications work through different mechanisms than fluoxetine and are specifically designed for weight management. Try our GLP-1 weight loss calculator to explore how these medications might support your weight loss goals based on your current weight and BMI. Your GP can assess whether you're eligible and discuss which option may be suitable for you.
- 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.
Does fluoxetine cause weight loss in everyone?
No. Weight responses to fluoxetine vary widely. Some people lose weight initially, others gain weight, and some experience no change. The weight loss effect, if it occurs, is usually temporary. Fluoxetine is an antidepressant, not a weight loss medication, so weight change is an unpredictable side effect rather than a reliable outcome.
How much weight can fluoxetine cause you to lose?
There's no typical amount — it depends entirely on the individual and often varies over time. Some people may lose a few pounds in the first weeks, but this frequently reverses as the body adapts. Using fluoxetine for weight loss is not recommended by NHS guidelines. If weight loss is your goal, speak to your GP about evidence-based options.
What are the approved weight loss medications available on the NHS in the UK?
Three medications are licensed for weight management: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All three are prescription-only and require clinical assessment. Eligibility typically requires a BMI of 30 or above, or 27 or above with weight-related health conditions. Your GP can refer you to a specialist weight management service to assess suitability.
Can I get prescription weight loss medication on the NHS or do I have to go private?
Both options are available in the UK. NHS access is through specialist tier 3 weight management services — you'll need a referral from your GP, and availability varies by region. Private access is available through online consultations with UK-licensed prescribers. Waiting times and costs differ; your GP can discuss what's available in your area.
Should I stop taking fluoxetine to try to lose weight?
No — never stop or reduce antidepressant medication without discussing it with your GP first. Fluoxetine is prescribed for mental health reasons, and stopping it can be harmful. If weight management is a concern, talk to your GP about your options. They can assess whether your current medication is contributing to weight gain and discuss evidence-based weight loss approaches alongside your mental health treatment.
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.