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 Mounjaro Lower Testosterone?
There is no clinical evidence that Mounjaro (tirzepatide) directly lowers testosterone levels. Testosterone changes are not listed as a known side effect of this prescription weight loss injection in UK medical guidance or clinical trials. However, weight loss itself can affect hormone levels, and rapid weight changes may influence testosterone temporarily. This guide explains what we know about Mounjaro, hormone health, and when to speak to your GP about concerns.
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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.
What Clinical Evidence Shows About Mounjaro and Hormones
Mounjaro is a dual GIP and GLP-1 receptor agonist approved in the UK for weight management in adults. The SURMOUNT clinical trials, which tested tirzepatide in thousands of participants, did not report testosterone reduction as a side effect. The MHRA and NICE have reviewed extensive trial data, and testosterone changes are not listed in the official prescribing information or safety profile.
That said, weight loss itself can affect hormone levels. Testosterone is produced partly in body fat tissue (aromatase converts other hormones to oestrogen in fat). gradual weight management may temporarily shift hormone balance. Additionally, improvements in insulin sensitivity from weight loss can normalise hormonal function in people with type 2 diabetes or metabolic syndrome. These changes are typically beneficial.
- No direct mechanism of tirzepatide affects testosterone production
- Weight loss may support healthier hormone levels overall
- Individual responses vary based on underlying health
- Speak to your GP if you notice unusual symptoms
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.
When to Discuss Hormone Health With Your GP
If you experience changes in energy, mood, libido, or other symptoms while taking Mounjaro, tell your GP. These could relate to weight loss, improved metabolic health, or other factors—not necessarily the medication itself. Your GP can assess your individual situation, check hormone levels if needed, and confirm whether your symptoms are expected or require adjustment.
Before starting Mounjaro, disclose any existing hormone-related conditions (thyroid disease, low testosterone, hormonal cancers) to your prescriber. They will assess whether Mounjaro is suitable for you. Use our GLP-1 weight loss calculator to understand projected outcomes, and ask your healthcare team about any hormone-related concerns specific to your health profile.
- 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 Mounjaro cause low testosterone?
No. Testosterone reduction is not a documented side effect of Mounjaro in clinical trials or UK prescribing guidance. However, if you experience symptoms like fatigue or mood changes, discuss them with your GP—they may relate to weight loss, improved health, or other factors.
Does weight loss from Mounjaro affect hormones?
Weight loss can shift hormone balance, but typically in beneficial ways. Losing excess weight may improve insulin sensitivity, reduce oestrogen from fat tissue, and support healthier hormone levels. Your GP can monitor this if needed.
Should I tell my GP about existing hormone conditions before taking Mounjaro?
Yes, absolutely. If you have thyroid disease, low testosterone, or hormone-related conditions, disclose this before starting Mounjaro. Your prescriber will assess whether it's suitable and may monitor you more closely.
What are the actual side effects of Mounjaro in the UK?
Common side effects include nausea, diarrhoea, constipation, and stomach pain—especially during dose titration. Serious side effects are rare but include pancreatitis and allergic reactions. See your GP if side effects concern you.
How does Mounjaro work, and could it affect testosterone?
Mounjaro activates GIP and GLP-1 hormone receptors to reduce appetite and support weight loss. It has no direct effect on testosterone production or hormone pathways that regulate testosterone. Weight loss benefits may support overall hormone health.
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.