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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Does Mounjaro Affect Your Period?
Mounjaro (tirzepatide) may affect menstrual cycles in some women, though it's not a direct side effect of the medication itself. Weight loss from Mounjaro can influence hormone levels and menstrual regularity. This guide explains how the medication and weight changes may impact your period, what to expect, and when to speak with your GP. Understanding these connections helps you make informed decisions about 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.
How Weight Loss Affects Menstruation
Significant weight loss—the primary effect of Mounjaro—can alter menstrual cycles because body fat produces oestrogen, a key hormone regulating periods. gradual weight management may cause temporary changes including irregular periods, lighter periods, or temporary amenorrhoea (missed periods). These changes are generally reversible once weight stabilises. In clinical trials, participants lost an average of 15–21% body weight, which could trigger hormonal shifts. Any substantial weight loss—whether from Mounjaro, diet, or exercise—can affect menstruation. If you're using hormonal contraception, including the combined oral contraceptive pill, Mounjaro may reduce its absorption due to slower stomach emptying, requiring additional contraceptive precautions for 4 weeks after starting and after each dose increase. Speak with your GP or prescriber if you're concerned about contraceptive effectiveness or unexpected menstrual changes.
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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.
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Mounjaro vs Wegovy: Menstrual Side Effects Comparison
Both Mounjaro (tirzepatide, a dual GIP and GLP-1 receptor agonist) and Wegovy (semaglutide, a GLP-1 agonist only) can indirectly influence menstrual cycles through weight loss rather than direct hormonal effects. The SURMOUNT-1 trial for Mounjaro and the STEP-1 trial for Wegovy did not identify menstrual changes as a primary side effect, though both medications documented nausea, diarrhoea, and appetite suppression. Individual responses vary significantly. Mounjaro's slightly higher average weight loss (approximately 20.9% at the 15 mg dose versus approximately 14.9% for Wegovy at 2.4 mg) might theoretically cause more pronounced menstrual changes in some women, but prescriber assessment determines which medication suits you individually. Your GP or prescriber will choose based on your medical history, contraceptive needs, and personal circumstances.
- 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 stop your period?
Mounjaro itself doesn't directly stop menstruation, but significant weight loss can cause temporary amenorrhoea (missed periods) in some women. This is a normal physiological response to rapid weight change and typically reverses once weight stabilises. If you miss more than two consecutive periods, consult your GP.
Will my period return to normal after stopping Mounjaro?
Yes, in most cases menstrual cycles normalise once weight stabilises and hormone levels adjust. This typically takes a few months after stopping the medication. However, individual timescales vary. If your period doesn't return to normal, speak with your GP.
Does Mounjaro make periods heavier or lighter?
Weight loss from Mounjaro may cause lighter periods or temporary irregular bleeding due to changes in oestrogen levels. Some women experience heavier periods initially during weight loss, then lighter ones as weight stabilises. These changes are individual and usually temporary. Track your cycle and report significant changes to your GP.
Can I take the combined pill with Mounjaro?
Yes, but with an important caveat: Mounjaro slows stomach emptying, which may reduce oral contraceptive absorption. The MHRA advises using additional contraception (e.g., condoms) for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. Discuss contraceptive options with your GP or prescriber.
Is Mounjaro safe if I'm trying to conceive?
Mounjaro is not recommended during pregnancy or if you're actively trying to conceive. If you're planning pregnancy, discuss timing with your GP. Moderate weight loss before conception can improve fertility outcomes, but Mounjaro should be stopped once you know you're pregnant.
Ready to take control of your weight loss journey?
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