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?
Whether Mounjaro (tirzepatide) affects your periods is an important question if you're considering this prescription weight management injection. The evidence suggests that Mounjaro itself does not directly cause menstrual changes, but gradual weight management from any cause — including weight loss medication — may temporarily affect your cycle. Understanding how tirzepatide works and what clinical evidence shows can help you make an informed decision with your GP or prescriber before starting treatment.
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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 Clinical Evidence Shows About Mounjaro and Periods
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — a prescription-only injection that works by activating natural hormone pathways related to appetite regulation and blood sugar control. It does not directly interact with oestrogen, progesterone, or other reproductive hormones.
In the SURMOUNT clinical trials, menstrual changes were not listed as a common or very common side effect of tirzepatide itself. However, gradual weight management from any cause — including weight loss medication — may temporarily affect your menstrual cycle. This can happen because:
- Significant weight loss can trigger temporary hormonal shifts that affect ovulation
- Low body fat percentage can suppress oestrogen production
- Rapid metabolic changes may temporarily disrupt the hypothalamic-pituitary-ovarian axis
These changes are usually temporary and typically resolve once your body adjusts or weight stabilises. Always discuss any menstrual changes with your GP, as they may indicate other health factors worth investigating.
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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.
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How to Monitor Your Cycle and When to Contact Your GP
If you're taking Mounjaro or considering it, track any changes to your menstrual cycle — including missed periods, heavier or lighter bleeding, or changes in cycle length. Keep a simple record and mention these at your next GP appointment or clinical review.
Contact your GP if:
- You miss a period entirely (pregnancy test may be needed)
- Your periods become significantly heavier or painful
- Menstrual changes persist beyond 3–4 months of stable weight
- You have other symptoms like abnormal bleeding or pelvic pain
Your prescriber needs to know about any side effects so they can assess whether Mounjaro is right for you or if adjustments are needed. Use our free GLP-1 weight loss calculator to track your projected weight loss and understand the pace of change your body may experience.
- 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?
Mounjaro itself is not listed as causing irregular periods in clinical trials. However, gradual weight management from any cause — including tirzepatide — may temporarily affect your menstrual cycle due to hormonal shifts. These changes are typically temporary and usually resolve as your body adjusts.
Will I need contraception while taking Mounjaro?
Yes. Mounjaro may reduce the absorption of oral contraceptives. If you take the combined pill or progestin-only pill, your prescriber will advise you to use additional barrier contraception or switch to a non-oral method for 4 weeks after starting Mounjaro and 4 weeks after each dose increase.
Is Mounjaro safe if I'm trying to conceive?
Mounjaro is not recommended if you're trying to become pregnant or are already pregnant. If you're planning pregnancy, discuss with your GP at least 3 months before stopping Mounjaro. Your prescriber can advise on timing and any pre-conception support you may need.
Could missing my period on Mounjaro mean I'm pregnant?
A missed period could indicate pregnancy, and a pregnancy test is recommended. However, it could also be a temporary effect of gradual weight management. Take a pregnancy test and contact your GP for advice — they may need to pause Mounjaro if pregnancy is confirmed.
How long do menstrual changes usually last on Mounjaro?
If menstrual changes occur due to weight loss, they're typically temporary and often resolve within 3–4 months once your weight stabilises. However, individual experiences vary. Keep a record and discuss persistent changes with your GP, as they may reflect other health factors worth investigating.
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.