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 Affect Your Menstrual Cycle?
Mounjaro (tirzepatide) may affect your menstrual cycle in some women, though not everyone experiences changes. Weight loss itself can influence hormone levels and period regularity, making it sometimes difficult to distinguish between effects caused by the medication and those caused by losing weight. If you're taking Mounjaro and notice changes to your cycle, it's important to understand what might be happening and when to seek medical advice from your GP.
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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.
How Weight Loss Affects Your Period
Significant weight loss can naturally impact menstrual cycles, regardless of whether you're taking Mounjaro or making lifestyle changes alone. gradual weight management or reaching a very low body fat percentage can disrupt hormones that regulate your period, potentially leading to irregular cycles, lighter periods, or temporary cessation of menstruation (amenorrhoea). The NHS recognises that weight is one of many factors affecting reproductive hormone balance. When you lose weight with Mounjaro, your body experiences hormonal shifts as fat stores decrease — fat tissue produces oestrogen, so significant fat loss can alter oestrogen levels. This is a normal physiological response and often reverses once weight stabilises. However, because Mounjaro helps you lose weight more quickly than diet and exercise alone, these hormonal changes may happen more rapidly, making period changes more noticeable.
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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.
Mounjaro, Oral Contraceptives, And Your Cycle
If you take oral contraceptives (the combined pill or progestin-only pill), you should be aware that Mounjaro may reduce how well your contraceptive is absorbed. The prescribing information specifically recommends adding a barrier method (condoms) or switching to a non-oral contraceptive for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This is because Mounjaro slows gastric emptying — how quickly food (and medications) move through your stomach — which can affect medication absorption. If you're relying on oral contraception for pregnancy prevention, discuss alternative contraception options with your GP or sexual health clinic before starting tirzepatide. Your prescriber will have reviewed this when assessing your eligibility.
- 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 irregular periods?
Weight loss — whether from Mounjaro or lifestyle changes — can affect menstrual regularity. Mounjaro may contribute indirectly through gradual weight management, which influences hormone levels. However, not all women experience period changes. If irregularities persist beyond a few months or cause concern, discuss with your GP.
Will my period return to normal if I stop Mounjaro?
Yes, in most cases. Once weight stabilises and hormonal balance is restored, menstrual cycles typically normalise. However, this varies individually. Your GP can provide personalised advice based on your health history.
Does Mounjaro affect hormone levels directly?
Mounjaro is not a hormone medication — it activates GLP-1 and GIP receptors to regulate appetite. It doesn't directly alter reproductive hormones. Period changes are primarily linked to weight loss effects on oestrogen and other hormones produced by fat tissue.
Can I take Mounjaro if I'm trying to conceive?
No. Mounjaro is not recommended during pregnancy or if you're planning to become pregnant. You should discuss family planning with your GP or prescriber before starting tirzepatide. Weight loss before conception may benefit fertility, but Mounjaro specifically is not suitable during this time.
What should I tell my GP about period changes on Mounjaro?
Report any significant changes — missed periods, much heavier or lighter bleeding, or severe pain. Also mention any other symptoms like fatigue or mood changes. Your GP may recommend a review of your treatment, dose, or overall health. Keep a simple record of changes to discuss.
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.