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 Period?
Yes, Mounjaro (tirzepatide) may affect your menstrual cycle in some cases. Weight loss itself, rapid changes in body weight, hormonal shifts, and stress related to injectable medications can all influence period regularity. Most changes are temporary, but it's important to understand what to expect and when to contact your GP. This guide explains how Mounjaro may impact your period and what you should monitor.
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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 Your Menstrual Cycle
Mounjaro works as a dual GIP and GLP-1 receptor agonist, helping regulate appetite and support weight loss alongside a reduced-calorie diet and increased physical activity. When you lose weight—particularly gradual weight management—your body's hormonal balance can shift.
- Oestrogen and body fat: Oestrogen is stored in adipose (fat) tissue. As you lose weight, oestrogen levels can fluctuate, which may cause irregular periods, lighter periods, or temporary cessation of menstruation.
- Stress and metabolic changes: gradual weight management can trigger physiological stress, which affects the hypothalamic-pituitary-ovarian axis—the system that regulates your menstrual cycle.
- Nutritional factors: If calorie restriction is too severe, deficiencies in iron, vitamin B12, or other nutrients may contribute to cycle irregularity.
These changes are usually temporary and often resolve as your body adjusts to its new weight. However, if your period stops completely for more than three months, contact your GP.
Clinically BackedWeight Loss
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Side Effects Related to Mounjaro and Periods
The Mounjaro KwikPen product information and clinical trial data do not list menstrual irregularity as a direct side effect of tirzepatide itself. However, nausea, vomiting, and reduced appetite—which are common Mounjaro side effects—may indirectly affect your nutrition and hormonal balance. Additionally, if you're taking oral contraceptives alongside Mounjaro, you should be aware that Mounjaro may reduce the absorption of oral medications for up to 4 weeks after starting and for 4 weeks after each dose titration. If this affects your contraceptive effectiveness, your prescriber may recommend switching to a barrier method or a non-oral contraceptive during these periods. Always inform your healthcare team about all medications and any period 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.
Will Mounjaro stop my period?
Mounjaro itself is not known to stop periods. However, gradual weight management can temporarily disrupt your menstrual cycle. Most people experience normalisation within a few months as their body adjusts. If your period stops completely for longer than three months, contact your GP to rule out other causes.
Is it normal to have irregular periods on Mounjaro?
Yes, period changes are not uncommon when losing weight with any method, including Mounjaro. Light periods, skipped periods, or changes in cycle length can occur due to hormonal shifts related to weight loss. Keep track of changes and discuss them with your GP if you're concerned.
Can Mounjaro affect my contraception?
Yes. Mounjaro may reduce the absorption of oral contraceptives for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. If you take the pill, discuss alternative contraception methods with your prescriber—such as a barrier method, patch, or implant—during these periods.
When should I contact my GP about period changes?
Contact your GP if your period stops completely for more than three months, if you experience severe or unusual bleeding, or if period changes are causing you distress. Also report if you're taking oral contraceptives and notice any unexpected spotting or changes in efficacy.
Are period changes permanent when using Mounjaro?
No. Period irregularities related to weight loss are usually temporary. Most people's cycles return to normal within a few months as their weight stabilises. If irregularities persist after your weight stabilises, speak to your GP to explore other causes.
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.