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 Menstrual Cycle?
Mounjaro (tirzepatide) is a prescription-only weight management injection that may affect your periods for some women. While period changes aren't listed as common side effects in the UK product information, many women report changes to their menstrual cycle when taking the medication. Weight loss itself can influence hormones and period regularity, and this effect may be amplified by Mounjaro. Understanding what's normal and when to seek advice from your GP is important.
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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.
How Mounjaro May Affect Your Period
Mounjaro works as a dual GIP and GLP-1 receptor agonist, helping regulate appetite and support weight loss. While the product information doesn't specifically list menstrual changes as a common side effect, anecdotal reports from women using the medication suggest period irregularities do occur. This may happen for several reasons: gradual weight management itself can influence oestrogen levels and cycle regularity; Mounjaro slows gastric emptying which may affect nutrient absorption, potentially impacting hormonal balance; and the medication's effects on the digestive system can influence how oral contraceptives are absorbed — contraception effectiveness may be reduced during dose escalation.
If you take an oral contraceptive pill and start Mounjaro, UK prescribing guidance recommends using additional contraception (such as condoms) or switching to a non-oral method for 4 weeks after starting the medication and 4 weeks after each dose increase. Your GP or prescriber should discuss this with you at your initial appointment.
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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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Everything you need to understand your weight, explore medication options, and track your progress — completely free.
What Period Changes to Expect and When to Seek Advice
Some women experience skipped periods, lighter or heavier bleeding, or irregular cycles within the first few weeks of starting Mounjaro or increasing doses. These changes often settle within 2–3 months as your body adjusts to the medication and weight loss stabilises. However, contact your GP if you experience severe pain, unusually heavy bleeding that doesn't improve, or periods that stop completely beyond 3 months. Hormonal imbalances can have other underlying causes, and your GP needs to rule these out. Keep a simple record of your cycle changes — this helps your GP assess whether the pattern is likely related to Mounjaro or warrants further investigation.
- 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 definitely affect my periods?
No. Not all women experience period changes on Mounjaro. Some have no change at all, whilst others report irregularities, lighter bleeding, or skipped periods. Individual responses vary significantly. Your prescriber should discuss this possibility with you before starting.
Is it safe to take Mounjaro if you have irregular periods?
Yes, but inform your GP or prescriber of any pre-existing menstrual irregularities. Mounjaro may worsen existing irregular cycles, or your doctor may want to investigate other hormonal causes first. A clinical assessment will determine suitability.
If I'm on the contraceptive pill, do I need extra protection with Mounjaro?
Yes. UK prescribing guidance recommends additional contraception (like condoms) or switching to a non-oral method for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. This is because Mounjaro slows digestion and may reduce pill absorption.
How long do period changes usually last?
Most women find their cycles stabilise within 2–3 months as the body adjusts and weight loss plateaus. However, if changes persist beyond 3 months or worsen, contact your GP for further assessment and reassurance.
Should I stop Mounjaro if my periods change?
Do not stop without medical advice. Contact your GP or prescriber first. Most menstrual changes are temporary and manageable. Your GP can advise whether continuation is appropriate or if any other action is needed based on your individual situation.
Ready to take control of your weight loss journey?
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