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 Mounjaro Affect Fertility?
There is no clinical evidence that Mounjaro (tirzepatide) causes infertility in men or women. However, the MHRA has issued important guidance: women of reproductive age should use reliable contraception whilst taking Mounjaro and for 4 weeks after stopping. This precaution exists because the medication may affect how the stomach absorbs oral contraceptives, not because Mounjaro itself damages fertility. If you're planning pregnancy or concerned about fertility, speak to your GP before starting tirzepatide.
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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 the Evidence Actually Shows
Clinical trials and real-world data show no direct link between tirzepatide and infertility. The SURMOUNT trials, which tested Mounjaro in thousands of adults, did not report infertility as a side effect. Weight loss itself often improves fertility in both men and women by reducing insulin resistance, lowering inflammation, and restoring hormonal balance.
- No clinical evidence of permanent fertility damage from GLP-1 or GIP/GLP-1 agonists
- Weight loss via Mounjaro may improve menstrual regularity and ovulation in women with polycystic ovary syndrome (PCOS)
- In men, modest weight loss can improve sperm quality and testosterone levels
- The contraception guidance is precautionary, not because the drug causes infertility
If you're taking or considering Mounjaro and planning to conceive, discuss your individual circumstances with your GP or fertility specialist.
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.
MHRA Guidance on Contraception
The UK's MHRA approved Mounjaro with specific guidance: women of childbearing potential should use effective, non-oral contraception (not the pill alone) during treatment and for 4 weeks after the final injection. This is because tirzepatide slows gastric emptying—how quickly food leaves the stomach—which may reduce how well oral contraceptives are absorbed. This does not mean Mounjaro causes infertility; it means oral contraceptives may be less effective. Options include the implant, IUD, barrier methods, or non-oral medications. Always inform your prescriber if you're planning pregnancy or currently trying to conceive.
- 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 cause infertility or permanent fertility damage?
No. Clinical trials found no link between tirzepatide and infertility. The medication does not damage sperm, eggs, or reproductive organs. Weight loss via Mounjaro may actually improve fertility in some people by reducing insulin resistance and hormonal imbalance.
Why does the MHRA say to use contraception with Mounjaro?
The guidance exists because Mounjaro slows stomach emptying, which may reduce how well oral contraceptive pills are absorbed. This makes the pill less effective—not because Mounjaro damages fertility. Barrier methods or long-acting contraceptives (implant, IUD) are unaffected.
Can I take Mounjaro if I'm trying to get pregnant?
Mounjaro is not recommended during pregnancy because safety data in pregnant women is limited. If you're trying to conceive, discuss with your GP or fertility specialist before starting. Weight loss beforehand may improve fertility outcomes.
Will Mounjaro affect my male partner's fertility?
No. Mounjaro in one partner does not affect the other's fertility. However, if your partner is overweight, modest weight loss may improve his sperm quality and fertility.
What should I do if I become pregnant whilst taking Mounjaro?
Stop Mounjaro and contact your GP or midwife immediately. Pregnancy is not recommended whilst on this medication. Your healthcare team will advise on safe next steps and any impact on your pregnancy.
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.