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 Mess With Your Period?
Yes, Mounjaro (tirzepatide) can affect your menstrual cycle in some women. Changes to your period are not listed as a common side effect in the official patient information, but period irregularities have been reported by some users. This may happen because gradual weight management itself can temporarily disrupt hormonal balance. If you experience significant changes to your period while taking Mounjaro, it's important to discuss this with your GP. They can assess whether the changes are related to the medication, your weight loss, or another underlying factor. Always report any new or unusual symptoms to your prescriber during 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
Mounjaro works as a dual GIP and GLP-1 receptor agonist, which regulates appetite and slows gastric emptying. gradual weight management from any source—including tirzepatide—can temporarily affect hormone levels, particularly oestrogen, which is stored in body fat. When fat cells shrink quickly, oestrogen is released into the bloodstream, potentially causing:
- Lighter or heavier periods
- Missed or delayed periods (amenorrhoea)
- Irregular cycle length
- Changes in period duration
- Increased or decreased menstrual cramping
These changes are often temporary and typically stabilise as your body adjusts to your new weight. However, every person responds differently. If you're considering Mounjaro and have concerns about your menstrual health, discuss this with your GP or prescriber before starting treatment, especially if you have irregular cycles, polycystic ovary syndrome (PCOS), or other hormonal conditions.
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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.
Mounjaro, Contraception & Period-Related Concerns
Important contraception note: Mounjaro can reduce how well oral contraceptives work because it slows how quickly food—and therefore medications—leave your stomach. If you take the contraceptive pill, you should add a barrier method (condoms) or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This is a critical safety consideration when assessing your Mounjaro eligibility UK with your prescriber.
If you're experiencing period changes while on Mounjaro, don't stop taking the medication—contact your GP or prescriber instead. They can help determine whether the changes are temporary hormone adjustment, weight-loss related, or warrant further investigation. Period irregularities usually resolve once your weight stabilises.
- 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 irregularity a known side effect of Mounjaro?
Period irregularities are not listed as a common side effect in the official product information for Mounjaro. However, they have been reported by some users and are likely linked to gradual weight management rather than the medication itself. Weight loss causes hormonal shifts, particularly involving oestrogen, which can temporarily disrupt your cycle. Report any changes to your GP.
Will my period return to normal once I stop Mounjaro?
In most cases, yes. If your period changes were caused by weight loss during Mounjaro treatment, your cycle typically stabilises once your weight plateaus. However, individual responses vary. If irregularities persist after stopping the medication, discuss this with your GP to rule out other causes.
Can I take Mounjaro if I have irregular periods or PCOS?
Yes, but you should discuss this with your GP or prescriber first. If you have polycystic ovary syndrome (PCOS) or other hormonal conditions, weight loss from Mounjaro may actually help improve some symptoms, though it can temporarily worsen others. Your prescriber will assess whether Mounjaro is appropriate for your individual situation.
What should I do if my period stops completely while on Mounjaro?
Contact your GP immediately. While temporary amenorrhoea (missed periods) can occur with gradual weight management, it's important to rule out other causes, including pregnancy complications or underlying hormonal issues. Your GP can assess whether this is weight-loss related or requires further investigation.
Does the dose of Mounjaro affect my period more than lower doses?
Higher doses of Mounjaro may cause more significant weight loss, which could theoretically increase the likelihood of period changes. However, the relationship between dose titration and menstrual changes hasn't been formally studied. Any period changes should be reported to your prescriber, who can assess your individual Mounjaro dosing plan.
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.