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
Mirtazapine and Weight Loss — What You Need to Know
Mirtazapine is an antidepressant that often causes weight gain as a side effect, not weight loss. If you're taking mirtazapine and concerned about your weight, you have several options. The UK has licensed weight loss medications available through the NHS and private prescribing. This guide explains the difference between antidepressant side effects and prescription weight management options, and how to find support if you're struggling with weight while on mental health treatment.
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.
Licensed Weight Loss Options Available in the UK
If mirtazapine is affecting your weight, speak to your GP before stopping the medication — they may suggest alternatives or additional support. The UK currently has three prescription-only weight management injections licensed for eligible adults: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide).
Eligibility typically requires a BMI of 30 or above, or BMI 27–29.9 with a weight-related health condition such as type 2 diabetes, high blood pressure, or sleep apnoea. Use our free BMI calculator above to check your current BMI and understand where you stand. Clinical trials show Mounjaro and Wegovy may help with meaningful weight loss — on average 15–21% of body weight — when used alongside dietary and lifestyle changes. Individual results vary significantly.
Both medications work as GLP-1 receptor agonists (Mounjaro is a dual GIP/GLP-1 agonist), helping reduce appetite and support feeling fuller after meals.
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.
How to Access Weight Loss Support in the UK
Access to prescription weight loss medication in the UK comes through two routes: NHS specialist services or private prescribing. NHS access is through tier 3 specialist weight management services, usually requiring a GP referral — though waiting lists vary significantly by region and can be lengthy. Private prescribing is available through GPhC-registered online pharmacies following an online clinical assessment; this route does not require NHS referral.
Your GP can discuss which option suits you and whether you're eligible. Try our GLP-1 weight loss calculator to estimate potential outcomes at different doses. Remember, weight loss medications work best alongside a reduced-calorie diet and increased physical activity — they are not a substitute for lifestyle change.
- 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 mirtazapine cause weight loss?
No. Mirtazapine is known to cause weight gain in most users — it increases appetite and can slow metabolism. If you're gaining weight on mirtazapine, discuss this with your GP. They may suggest an alternative antidepressant, adjust your dose, or recommend additional weight management support. Do not stop mirtazapine without medical guidance.
What weight loss medications are available on the NHS in the UK?
The NHS prescribes Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide) for eligible adults through specialist tier 3 weight management services. Access requires referral from your GP and assessment by a specialist team. Eligibility is based on BMI and weight-related health conditions. Waiting times vary by region.
Can I take weight loss medication whilst on mirtazapine?
This depends on your individual health and medications. Always inform any prescriber (GP or private) that you take mirtazapine before starting weight loss medication. Some GLP-1 medications may interact with antidepressants or affect how your body absorbs oral medicines. A healthcare professional must assess your safety before prescribing.
What is the difference between Mounjaro and Wegovy for weight loss?
Both are once-weekly injections licensed for weight management in the UK. Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist; Wegovy (semaglutide) is a GLP-1-only agonist. Clinical trials show similar average weight loss (15–21% body weight) but individual responses vary. Your prescriber will recommend which suits you best based on your health profile.
How do I find out if I'm eligible for weight loss medication in the UK?
Calculate your BMI using our free BMI calculator — you're typically eligible if BMI is 30+ or 27–29.9 with weight-related health conditions (diabetes, high blood pressure, sleep apnoea). Speak to your GP about referral to specialist weight management services, or explore private prescribing options through registered online healthcare providers. Both require a clinical assessment.
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.