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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Can Mounjaro Affect Your Period? What You Need to Know
Mounjaro (tirzepatide) may affect your menstrual cycle as an indirect side effect, though this is not commonly reported. Weight loss medications can influence hormone levels and menstrual regularity in some people. This guide compares Mounjaro and Wegovy (semaglutide) in terms of menstrual effects, explains the mechanisms behind potential changes, and provides evidence-based information to help you make informed decisions with your prescriber. Always consult your GP before starting any weight loss medication.
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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 GLP-1 and Tirzepatide Affect the Body — STEP vs SURMOUNT Context
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) activates only the GLP-1 pathway. This mechanical difference means they may affect the body differently, though both slow gastric emptying and reduce appetite. Neither medication directly targets reproductive hormones. However, both can cause weight loss and rapid changes to blood sugar levels, which may indirectly influence menstrual regularity in some women.
In the SURMOUNT-1 trial (Mounjaro), menstrual changes were not commonly reported as a significant side effect. Similarly, the STEP trials (Wegovy) did not highlight menstrual disruption as a primary concern. Weight loss itself—rather than the medication mechanism—is often the factor affecting periods, as significant body composition changes can influence oestrogen levels stored in fat tissue.
- Both medications work via appetite and metabolic pathways, not hormonal pathways
- gradual weight management (not the drug itself) most commonly affects menstrual cycles
- Individual responses vary significantly
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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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Menstrual Changes, Weight Loss, and Your Prescriber's Assessment
If you experience irregular periods, changes to cycle length, or heavier/lighter menstruation while on Mounjaro or Wegovy, this is important information for your GP or prescriber. Menstrual changes are not necessarily dangerous, but they warrant discussion because they indicate metabolic shifts. Women taking oral contraceptives should note that both medications may reduce the absorption of oral medications for up to 4 weeks after starting treatment and after each dose increase—switching to a barrier method or non-oral contraceptive is recommended during these periods.
Your prescriber will assess whether either medication is suitable for your individual health, including reproductive health. Use our free GLP-1 weight loss calculator to explore potential outcomes, and try the BMI calculator to understand your personal weight category—both tools help inform your conversation with your prescriber about suitability and expected changes.
- 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 directly affect my hormones and period?
Mounjaro does not directly target reproductive hormones. However, gradual weight management and changes to blood sugar levels may indirectly affect menstrual regularity. Most menstrual changes are linked to weight loss itself rather than the tirzepatide mechanism. Always report menstrual changes to your prescriber.
Is Wegovy more likely than Mounjaro to affect my period?
Clinical trial data (STEP vs SURMOUNT) did not highlight menstrual disruption as a common side effect for either medication. Individual responses vary. Both work via GLP-1 pathways, though Mounjaro's dual mechanism may affect the body slightly differently—but neither directly influences reproductive hormones.
Should I stop my contraceptive pill if I start Mounjaro?
Do not stop without consulting your GP. Both Mounjaro and Wegovy may reduce oral medication absorption for 4 weeks after starting and after each dose increase. Your prescriber may recommend adding a barrier method or switching to a non-oral contraceptive during these periods—discuss this during your clinical assessment.
When will my period return to normal after stopping Mounjaro?
Menstrual regularity typically returns as your weight stabilises and weight loss slows. This can take several weeks to months depending on how much weight you lost and how quickly. If irregular periods persist after stopping, discuss this with your GP to rule out other causes.
Are menstrual changes a sign Mounjaro isn't working or is unsafe?
Menstrual changes are a sign your body is undergoing metabolic shifts, often due to weight loss. They are not necessarily unsafe, but they are important to monitor. Report all changes to your prescriber. They can assess whether the medication remains appropriate for you based on your overall health.
Ready to take control of your weight loss journey?
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