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
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Mounjaro and Menstrual Changes — Comparing Mounjaro vs Wegovy
Missing or irregular periods can be a concerning side effect when using Mounjaro (tirzepatide) for weight management. This page compares how Mounjaro and Wegovy affect menstrual cycles, explains the mechanisms behind these changes, and helps you understand when to seek GP guidance. Both medications are prescription-only GLP-1 or dual GIP/GLP-1 receptor agonists, but they work slightly differently and may have different individual effects. If your periods have changed since starting either medication, this guide will help you understand what's happening and what questions to ask your healthcare provider.
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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 and Wegovy Affect Menstrual Cycles — Clinical Context
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single pathway. Both medications can affect menstrual cycles, though the exact mechanisms are not fully understood clinically.
Changes in menstrual function may occur due to:
- gradual weight management itself — significant weight loss can temporarily disrupt hormone balance and menstrual regularity
- GLP-1 receptor activation in the hypothalamus and pituitary gland, which regulate reproductive hormones
- Changes in body fat percentage — oestrogen is produced in fat tissue, so rapid fat loss can affect cycle regularity
- Stress hormones — changes in cortisol during gradual weight management
In clinical trials (SURMOUNT for Mounjaro, STEP for Wegovy), menstrual irregularities were reported but were not the primary focus of data collection, so direct comparison is limited. Individual variation is significant — some people experience no change, whilst others report missed or irregular periods.
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What to Do If Your Period Changes — When to Contact Your GP
If you notice missed or irregular periods after starting Mounjaro or Wegovy, this does not necessarily mean you should stop the medication immediately — but it does warrant discussion with your GP or prescriber.
Contact your GP if:
- Your period is absent for more than 3 months
- You experience sudden heavy bleeding or severe pain
- You're concerned about pregnancy (use barrier contraception or non-oral methods as these medications may reduce oral contraceptive absorption)
- Irregular periods persist beyond the initial adjustment phase
Your GP can assess whether the menstrual change is related to the medication, the weight loss itself, or another underlying cause. Some people find that menstrual regularity returns as weight stabilises, whilst others may need to adjust medication or dose. Use our free BMI calculator and progress tracker to monitor both your weight and health changes alongside any symptoms you 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.
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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.
Is a missed period a common side effect of Mounjaro?
Menstrual irregularities including missed periods have been reported by some people taking Mounjaro, but they are not listed as "very common" side effects in clinical trials. Individual experiences vary widely. gradual weight management itself can affect menstrual regularity independent of the medication. If this concerns you, discuss it with your GP.
Why do GLP-1 medications like Wegovy affect periods?
The exact mechanism isn't fully understood, but GLP-1 receptors are present in areas of the brain that regulate reproductive hormones. Additionally, gradual weight management reduces body fat, which produces oestrogen. This hormonal shift can temporarily disrupt menstrual cycles. Most people find regularity returns as weight stabilises.
Can I take the oral contraceptive pill while on Mounjaro?
Both Mounjaro and Wegovy can reduce the absorption of oral contraceptives due to slowed gastric emptying. The MHRA recommends using a barrier method or switching to a non-oral contraceptive for 4 weeks after starting and after each dose increase. Discuss contraception options with your GP or prescriber.
How long do menstrual changes last on Mounjaro or Wegovy?
There's no set timeframe — it varies individually. Some people experience temporary irregularities during gradual weight management that resolve within a few months. Others may have longer-term changes. If your period doesn't return to normal within 3 months, or if you're concerned, speak to your GP for reassurance and investigation.
Is a missed period a reason to stop Mounjaro or Wegovy?
Not necessarily. A single missed period is often not cause for immediate discontinuation, but it's worth reporting to your prescriber. Your GP can help determine whether the change is medication-related, weight-loss-related, or due to another cause, and can advise whether continuing, adjusting dose, or stopping is appropriate for you.
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