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
Can Mounjaro Affect Your Menstrual Cycle?
Yes, Mounjaro (tirzepatide) may affect periods in some women. Changes to menstrual cycles are not listed as common side effects in UK clinical data, but weight loss itself can influence hormonal balance and period regularity. If you're considering Mounjaro, it's important to discuss any concerns about your menstrual cycle with your GP or prescriber during your clinical assessment. They can review your individual health history and help you understand what to expect. Using our GLP-1 weight loss calculator can help you project potential outcomes, whilst our BMI calculator provides baseline health context before starting any weight loss 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.
Mounjaro vs Wegovy: Menstrual Side Effects Compared
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. In the SURMOUNT clinical trials, menstrual irregularities were not reported as a common or very common side effect, though weight loss itself can affect hormone levels and cycle regularity.
Wegovy (semaglutide) is a GLP-1 agonist only. Similarly, menstrual changes are not listed as common side effects in UK STEP trial data, though individual experiences vary.
Neither medication is directly documented as causing period disruption in clinical trials. However, gradual weight management of any kind—including from these injections—can trigger temporary hormonal changes that affect menstruation. This is a well-established physiological response, not a direct drug effect. Your prescriber will ask about your menstrual history during assessment and can advise on what's normal for you personally.
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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 You Should Tell Your Prescriber
Before starting Mounjaro or Wegovy, always mention:
- Irregular or heavy periods
- History of hormonal conditions (PCOS, endometriosis, thyroid disorders)
- Recent changes in menstrual pattern
- Contraceptive use (especially oral pills—Mounjaro may reduce their effectiveness)
- Plans to become pregnant or breastfeed
Your prescriber will assess whether these injections are suitable for you. If you experience significant menstrual changes after starting treatment, contact your GP or prescriber immediately. Track your weight loss progress using our free progress tracker whilst monitoring any changes to your cycle.
- 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.
Does Mounjaro directly cause period changes?
Mounjaro is not documented in UK clinical trials as directly causing menstrual irregularities. However, gradual weight management—from any cause—can temporarily affect hormone balance and period regularity. This is a normal physiological response, not a direct side effect of the medication itself.
Is Wegovy safer for periods than Mounjaro?
Both Mounjaro and Wegovy show similar menstrual safety profiles in clinical trial data. Neither lists menstrual changes as a common side effect. Individual responses vary. Your prescriber will assess which medication suits your personal health history during clinical assessment.
Will my periods return to normal after stopping Mounjaro?
If menstrual changes are due to weight loss rather than the medication directly, periods typically normalise as weight stabilises. If you stop Mounjaro, work with your GP on a plan to maintain weight loss and monitor cycle regularity. Hormone levels usually recover over weeks to months.
Can I take Mounjaro if I use the contraceptive pill?
Mounjaro may reduce oral contraceptive effectiveness because it slows stomach emptying and can reduce absorption of oral medications. If you use the pill, add a barrier contraceptive method or switch to a non-oral method for 4 weeks after starting Mounjaro and 4 weeks after each dose increase.
Should I delay starting Mounjaro if my periods are irregular?
Not necessarily, but inform your prescriber. They will review your medical history and menstrual pattern to determine if Mounjaro is appropriate for you. Some hormonal conditions (like PCOS) may benefit from weight loss, whilst others require careful monitoring during treatment.
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.