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 Your Period?
Yes, Mounjaro (tirzepatide) may affect your menstrual cycle, though this is not a direct effect of the medication itself. Instead, weight loss, stress, and hormonal changes caused by Mounjaro can influence your period. Some women report irregular periods, lighter periods, or temporary changes in cycle length whilst taking Mounjaro. If you experience significant menstrual changes, always speak to your GP, as they can assess whether it's related to Mounjaro or another underlying health factor.
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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 Menstrual Cycle
The primary reason Mounjaro may affect your period is gradual weight management. When you lose weight quickly — particularly 15–20% of body weight as seen in clinical trials — your body's hormonal balance shifts. Oestrogen is stored in fat tissue, so when fat decreases significantly, oestrogen levels can drop, which may cause:
- Irregular or delayed periods
- Lighter or heavier bleeding
- Temporary amenorrhoea (missed periods)
- Changes in cycle length
This effect is temporary in most cases and typically resolves once your weight stabilises. The NHS recognises that gradual weight management from any cause — whether through diet, exercise, or medication — can temporarily disrupt menstrual function. If you're concerned about ongoing changes, your GP can investigate whether other factors are involved.
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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 Side Effects That May Affect Your Cycle
Beyond weight loss, other Mounjaro side effects may indirectly influence your period. Stress and nausea — both common during the first few weeks of Mounjaro — can suppress menstrual function. Additionally, if Mounjaro causes you to lose appetite or eat significantly less, nutritional deficiencies may develop, which can affect hormone regulation.
The good news: these effects are usually temporary. Most women find their cycle returns to normal within 2–3 months as their body adjusts to the medication and weight loss stabilises. Always report significant menstrual changes to your GP, especially if they persist beyond 3 months or are accompanied by other concerning symptoms.
- 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.
Will Mounjaro stop my period completely?
Most women experience temporary changes rather than complete amenorrhoea. gradual weight management can delay or lighten periods, but complete cessation is rare unless weight loss is extreme. If your period stops entirely, contact your GP to rule out other causes.
Is it safe to take Mounjaro if I have irregular periods?
Yes, but inform your prescriber before starting treatment. They'll assess your individual situation. Some underlying hormonal conditions may require monitoring whilst on Mounjaro, so your GP should know your full menstrual history.
Should I use contraception differently whilst taking Mounjaro?
Yes. Mounjaro slows gastric emptying, which may reduce absorption of oral contraceptives. The NHS recommends adding a barrier method or switching to non-oral contraception for 4 weeks after starting Mounjaro and 4 weeks after each dose increase.
How long does it take for my period to return to normal after Mounjaro?
Most women see their cycle normalise within 2–3 months once weight loss slows and their body adjusts. If irregular periods persist beyond 3 months, speak to your GP to rule out other hormonal issues.
Can I use the MyWeightCheck GLP-1 calculator to understand weight loss and period changes?
Yes. Our free weight loss projection calculator helps you understand expected weight loss at different Mounjaro doses, which can help contextualise why menstrual changes occur during rapid loss phases.
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.