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
Does Mounjaro Affect Your Period?
Mounjaro (tirzepatide) can affect menstrual cycles in some people, though this isn't listed as a common side effect in clinical trials. Weight loss itself may cause temporary period changes, and rapid hormonal shifts from GLP-1 medications can influence cycle regularity. If you're considering Mounjaro on an NHS NICE-approved pathway or through private prescription, it's essential to discuss menstrual health with your prescriber during clinical assessment. Both Mounjaro and Wegovy may have similar effects, as weight loss is the primary driver of cycle changes rather than the medication itself.
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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.
Mounjaro, Wegovy, and Menstrual Changes — Trial Evidence
Clinical trial data from SURMOUNT (Mounjaro) and STEP (Wegovy) studies do not list irregular periods or amenorrhea as common side effects of either medication. However, both are GLP-1 receptor agonists (Mounjaro is dual GIP+GLP-1), and gradual weight management — averaging 15–21% body weight with Mounjaro and approximately 15% with Wegovy — can trigger menstrual irregularities.
Why weight loss affects periods:
- Hormonal changes from fat tissue loss (oestrogen is stored in fat)
- Rapid calorie deficit affecting reproductive hormone balance
- Stress on the hypothalamic-pituitary-ovarian axis during significant weight changes
Both medications work through appetite suppression and reduced calorie intake, so the mechanism triggering period changes is similar between Mounjaro and Wegovy. Individual responses vary significantly — some users experience no changes, whilst others notice lighter, heavier, or irregular cycles.
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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
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What to Do If Your Period Changes — Prescriber Guidance
If you experience period changes after starting Mounjaro or Wegovy, inform your GP or prescriber immediately. Report the specific change: lighter/heavier flow, missed periods (amenorrhea), longer or shorter cycles, or increased cramps. Your prescriber may adjust your dose, modify your calorie deficit, or refer you for further assessment if changes persist beyond 3 months.
Before starting either medication, mention if you have a history of irregular periods, hormonal contraceptive use, or conditions like polycystic ovary syndrome (PCOS). Your UK-licensed prescriber will assess whether the medication is appropriate and monitor your menstrual health. Use our free progress tracker to log not just weight changes, but also cycle regularity — this information is valuable for your healthcare team.
- 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 altogether?
Amenorrhea (missed periods) is possible with significant weight loss but is not a documented common side effect of tirzepatide specifically. If your period stops, contact your GP — this may indicate the weight loss rate is too rapid or hormonal balance needs reassessment. Temporary amenorrhea typically resolves once weight loss stabilises.
Is there a difference between Mounjaro and Wegovy for period effects?
Not significantly. Both are GLP-1-based medications (Mounjaro also has GIP activity), and menstrual changes are more linked to weight loss speed than the specific medication. Wegovy users report similar period changes in clinical practice. Individual responses vary — prescriber monitoring is essential.
Will my period return to normal after stopping Mounjaro?
Usually, yes. Once weight stabilises and hormonal balance recovers, most people experience menstrual regulation return within 1–3 months. However, if you stop taking Mounjaro, weight regain may occur without continued dietary changes. Discuss long-term management with your prescriber.
Should I use additional contraception if Mounjaro affects my cycle?
If taking oral contraceptives, know that Mounjaro may reduce absorption of oral medications for 4 weeks after starting and after each dose increase. Your prescriber will recommend adding barrier methods or switching contraception during these periods. Discuss all contraceptive options with your GP.
What dose of Mounjaro is most likely to cause period changes?
Period changes are linked to overall weight loss rate rather than dose level. However, higher maintenance doses (10–15 mg weekly via the KwikPen) may produce faster weight loss if combined with strict calorie restriction, increasing the risk. A balanced approach with moderate calorie deficit and prescriber support reduces this risk.
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.