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 Make Your Period Late?
Yes, Mounjaro may affect your menstrual cycle in some women. While period changes are not listed as very common side effects in the official product information, clinical experience and patient reports suggest that significant weight loss, nausea, and hormonal changes triggered by tirzepatide can occasionally cause irregular periods, delayed periods, or temporary amenorrhoea (absence of periods). If you experience changes to your menstrual cycle whilst taking Mounjaro, it's important to speak with your GP or the prescriber who issued your prescription to rule out other causes and discuss whether the medication may be responsible.
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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 Menstrual Cycle
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that works by reducing appetite and supporting weight loss. Several mechanisms may influence menstrual regularity:
- gradual weight management: Losing weight quickly can disrupt hormonal balance, particularly oestrogen levels, which regulate the menstrual cycle. This is especially likely if weight loss exceeds 1–1.5 kg per week.
- Nausea and reduced food intake: Mounjaro commonly causes nausea, which may lead to calorie deficit beyond your intended intake, accelerating weight loss and hormonal disruption.
- Stress on the body: Significant dietary changes and metabolic shifts can trigger temporary menstrual irregularity.
- Hormonal sensitivity: Some women's cycles are more sensitive to metabolic change than others.
Period disruption is usually temporary and often resolves once your body adjusts to the medication and weight loss stabilises. However, if you're concerned, your GP or prescriber can assess whether other factors might be involved.
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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.
When to Contact Your GP or Prescriber
Contact your GP or the healthcare professional who prescribed your Mounjaro if you experience:
- Periods that have stopped for three months or more (amenorrhoea)
- Significant changes to your cycle length or flow that persist beyond two injection cycles
- Severe period pain or unusual bleeding
- Concerns that the period change may indicate another health issue
- Questions about whether Mounjaro is the cause
Your prescriber may recommend continuing the medication if benefits outweigh concerns, adjusting your dose, or investigating other possible causes. Never stop taking Mounjaro without medical guidance, as this requires proper management. If you're taking oral contraceptives, remember that Mounjaro may reduce their absorption during the first 4 weeks after starting and after each dose increase—use additional contraception during these periods if you wish to avoid pregnancy.
- 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.
Is period delay a common side effect of Mounjaro?
Period irregularity is not listed as a very common side effect in the official Mounjaro information, but clinical reports and patient experience suggest it can occur in some women, particularly those experiencing gradual weight management. If it happens to you, contact your prescriber or GP to discuss.
Why might Mounjaro affect my periods?
gradual weight management and metabolic changes triggered by Mounjaro can disrupt hormonal balance, particularly oestrogen levels. Additionally, nausea from the medication may reduce food intake beyond your planned calorie deficit, accelerating weight loss and potentially affecting menstrual regularity.
Will my periods return to normal after stopping Mounjaro?
In most cases, menstrual cycles return to normal once Mounjaro is discontinued and your body adjusts to stable weight and nutrition. However, this varies individually. If you're considering stopping for this reason, discuss it with your prescriber first—do not stop without medical guidance.
What if I'm trying to get pregnant and taking Mounjaro?
Mounjaro is not recommended during pregnancy. If you're planning to conceive, you should stop Mounjaro and wait at least one menstrual cycle before attempting pregnancy. Period irregularity may actually make tracking ovulation more difficult—speak with your GP about timing if conception is your goal.
Can I take Mounjaro if I'm on the contraceptive pill?
Yes, but Mounjaro may reduce the absorption of oral contraceptives for 4 weeks after starting and 4 weeks after each dose increase. During these periods, use additional contraception (condoms, IUD) if you want to avoid pregnancy. Your prescriber will advise you on this when your prescription is issued.
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.