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) may affect your menstrual cycle as your body adjusts to the medication. This is not unusual when starting weight management treatment, as hormonal changes related to weight loss itself can influence menstruation. Most people experience cycle normalisation within a few months, though individual experiences vary significantly. If you have concerns about changes to your period whilst on Mounjaro, your GP can provide personalised guidance. Always inform your prescriber of any significant menstrual changes before making any decisions about your treatment.
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Calculate Your BMI Instantly
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
Weight loss itself impacts menstruation. When you lose weight — especially rapidly — your body's oestrogen levels can shift, which may cause irregular periods, lighter or heavier bleeding, or temporary cessation of menstruation (amenorrhoea). This is a natural physiological response, not a direct side effect of the tirzepatide medication itself.
Mounjaro works by reducing appetite and supporting a calorie deficit, meaning weight loss can occur more quickly than with diet and exercise alone. The faster the weight loss, the more likely hormonal adjustment occurs. Additionally, some people on Mounjaro report nausea or reduced appetite, which can affect overall nutrition — and nutritional status also influences menstrual regularity.
When menstrual changes occur:
- Most changes settle within 1–3 months as your body adapts
- Irregular cycles are typically temporary, not permanent
- Extremely heavy bleeding or severe pain should be reviewed by your GP
- Keep a brief log of changes to discuss with your prescriber at your next review
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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
Contact your GP if you experience very heavy bleeding, severe cramping, or complete absence of menstruation lasting more than 3 months — these warrant medical assessment to rule out other causes. You should also speak to your prescriber if menstrual changes are causing you distress or affecting your quality of life.
If you take oral contraceptives, remember that Mounjaro may reduce the absorption of oral medications for up to 4 weeks after starting and after each dose increase. Use a backup contraception method (condoms or IUD) during these periods to ensure reliable contraceptive cover. Your GP or prescriber can advise on the safest approach for your individual situation and whether timing adjustments might help.
- 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 an irregular period a side effect of Mounjaro itself?
No — Mounjaro doesn't directly cause menstrual changes. Instead, gradual weight management (which Mounjaro supports) triggers natural hormonal shifts in your body that can affect your cycle. This is temporary and usually settles as your weight stabilises.
How long do menstrual changes last on Mounjaro?
Most people experience cycle normalisation within 1–3 months. If changes persist beyond 3 months or are severe, contact your GP to rule out other causes and discuss whether any adjustments to your treatment plan might help.
Does Mounjaro affect birth control pills?
Yes — Mounjaro may reduce the absorption of oral contraceptives for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. Use a barrier method (condoms) or switch to a non-oral contraceptive like an IUD during these periods to ensure reliable protection.
Can I take Mounjaro if I have heavy periods?
Heavy periods are not a contraindication to Mounjaro, but your prescriber should know about them at assessment. If your periods become significantly heavier on Mounjaro, contact your GP — they can investigate and advise whether the medication should continue or be adjusted.
Will my period return to normal after stopping Mounjaro?
Yes — menstrual cycles typically return to their baseline pattern once you stop tirzepatide and your weight stabilises. If changes persist after stopping, speak to your GP to explore other possible 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.