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, though this isn't listed as a common side effect in the UK prescribing information. Weight loss itself can influence hormone levels and period regularity, making it difficult to determine whether changes are caused directly by Mounjaro or by the weight loss it supports. If you experience significant changes to your period whilst taking Mounjaro, it's important to discuss this with your GP or the healthcare professional managing your treatment.
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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 is a dual GIP and GLP-1 receptor agonist that works by regulating appetite and supporting weight loss. Whilst period changes aren't formally listed as a very common side effect in the UK MHRA prescribing information, several factors may influence your cycle:
- gradual weight management can temporarily affect oestrogen levels and cycle regularity — this is most noticeable in the first few months of treatment
- Stress and metabolic changes during significant weight loss may trigger irregular periods or missed cycles
- Gastrointestinal side effects like nausea or reduced food intake may affect nutrient absorption, which can influence menstrual regularity
- Individual variation — some people experience no changes, whilst others notice irregular periods that normalise over time
These changes are typically temporary and often resolve as your body adjusts to both the medication and your new weight. Using our free GLP-1 weight loss calculator can help you understand expected weight loss timelines, which may help contextualise any period changes you experience.
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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.
What to Do If Your Period Changes
If you notice significant changes to your menstrual cycle whilst taking Mounjaro — such as missed periods, much heavier or lighter bleeding, or unusually painful periods — contact your GP or the specialist managing your prescription. Don't stop taking Mounjaro without medical advice. Your healthcare team can:
- Assess whether changes are related to Mounjaro, weight loss, or another cause
- Check hormone levels if appropriate
- Adjust your treatment plan if needed
- Provide reassurance if changes are temporary
Keep a simple record of any changes to share with your GP — noting dates, flow changes, and when they started relative to starting or increasing your Mounjaro dose helps your healthcare team make an informed assessment.
- 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 a missed period a side effect of Mounjaro?
Missed periods aren't listed as a common side effect, but gradual weight management can temporarily disrupt menstrual cycles by affecting oestrogen levels. This is more often related to the weight loss itself rather than the medication directly. If you miss more than one period, contact your GP for assessment.
Will my period return to normal after starting Mounjaro?
In most cases, yes — menstrual irregularity related to weight loss typically normalises within a few months as your body adjusts. However, individual experiences vary. If changes persist beyond three months, discuss this with your healthcare provider.
Should I use additional contraception if Mounjaro affects my period?
Yes — Mounjaro may reduce the effectiveness of oral contraceptives because it slows stomach emptying and can reduce medication absorption. If you take the pill, add a barrier method or consider switching contraception. Your GP or prescriber can advise on alternatives.
Can Mounjaro cause heavy periods or spotting?
Whilst heavy bleeding isn't a documented side effect, gastrointestinal symptoms like nausea might affect nutrient absorption, and gradual weight management can influence hormone balance. Any unusual bleeding should be discussed with your GP to rule out other causes.
Should I tell my GP about period changes before starting Mounjaro?
Yes — mention any existing menstrual irregularities, heavy periods, or hormonal conditions when your prescriber assesses you for Mounjaro eligibility. This helps them monitor you for medication-related changes and distinguish them from your baseline cycle pattern.
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.