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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Does Mounjaro Lower Testosterone — What the Evidence Shows
There is currently no robust clinical evidence that Mounjaro (tirzepatide) directly lowers testosterone levels in men or women. In the SURMOUNT clinical trials, hormone changes were not a reported safety concern. However, weight loss itself can affect hormone levels, and gradual weight management may influence testosterone production in some individuals. This page compares Mounjaro and Wegovy (semaglutide) in terms of reported side effects, mechanism of action, and when to discuss hormone concerns with your GP. Both are prescription-only GLP-1 and GIP/GLP-1 medications used alongside reduced-calorie diet and increased physical activity for weight management in UK adults.
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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.
Mounjaro vs Wegovy — Clinical Evidence on Hormone Effects
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. In the SURMOUNT-1 clinical trial, the most commonly reported side effects were nausea, diarrhoea, vomiting, and constipation — not hormone-related. Hormone levels, including testosterone, were not identified as a primary safety signal in the trials published to date.
Wegovy (semaglutide) is a GLP-1 receptor agonist only. Similarly, in STEP clinical trials, reported side effects focused on gastrointestinal effects. Testosterone changes were not flagged as a significant safety concern. Both medications work by affecting appetite and satiety, not by directly interacting with hormone production pathways.
Weight loss can indirectly affect hormone balance — gradual weight management may cause fluctuations in testosterone, oestrogen, and other hormones due to changes in body composition and fat distribution. This is a general effect of significant weight loss, not specific to either medication. If you notice symptoms like fatigue, mood changes, or reduced libido, discuss these with your GP or prescriber.
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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.
Should You Discuss Hormone Concerns with Your Prescriber?
Yes. Before starting either Mounjaro or Wegovy, inform your prescriber about any existing hormone-related conditions, including hypogonadism, thyroid disorders, or hormonal contraceptive use. Women taking oral contraceptives should use additional barrier contraception for 4 weeks after starting Mounjaro and 4 weeks after each dose increase, as Mounjaro may reduce oral medication absorption.
Both medications are prescription-only and require clinical assessment. Your prescriber will review your full health history. If you experience symptoms like unusual fatigue, mood changes, or changes in sexual function during treatment, report these to your GP — they may be related to weight loss itself, underlying health conditions, or other medications you're taking. Use our GLP-1 weight loss calculator to estimate projected outcomes, and discuss your individual suitability with a healthcare professional.
- 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 Mounjaro directly affect testosterone production?
No clinical evidence suggests Mounjaro directly lowers testosterone. SURMOUNT trials did not identify hormone changes as a safety concern. However, significant weight loss can indirectly affect hormone balance due to changes in body composition. Discuss any hormone concerns with your GP.
Is Wegovy safer than Mounjaro for hormone levels?
Both Mounjaro and Wegovy have similar safety profiles regarding hormones — neither has robust evidence of directly affecting testosterone. Side effects reported in trials were primarily gastrointestinal. Individual responses vary, and prescriber assessment is essential before starting either medication.
Can weight loss from GLP-1 injections cause testosterone changes?
Yes, gradual weight management from any cause — including GLP-1 medication — may affect hormone levels due to changes in fat distribution and body composition. This is a general effect of weight loss, not specific to Mounjaro or Wegovy. NHS recommends losing 0.5–1 kg per week for sustainable results.
Should I tell my GP about hormone concerns before starting Mounjaro?
Absolutely. Inform your prescriber about any existing hormone disorders, thyroid conditions, or hormonal contraceptive use before starting. This ensures a proper clinical assessment and any necessary monitoring. Both medications are prescription-only and require individual suitability evaluation.
What are the most common Mounjaro side effects compared to Wegovy?
Both medications commonly cause nausea, diarrhoea, vomiting, and constipation. Mounjaro uses dual GIP+GLP-1 mechanism (up to 15 mg weekly), while Wegovy uses GLP-1 only (up to 2.4 mg weekly). Side effect profiles are similar. Your GP can advise which is more suitable for your circumstances.
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.