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 vs Wegovy
Both Mounjaro and Wegovy may affect your menstrual cycle, though this is not a commonly reported side effect in clinical trials. Weight loss itself can influence period regularity, and rapid weight changes may temporarily affect hormone levels. If you're considering either medication, understanding potential effects on your cycle is important for making an informed decision with your prescriber.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 and Wegovy Differ in Mechanism and Effect
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways rather than one. Wegovy (semaglutide) is a GLP-1-only agonist. This difference in mechanism means they may have subtly different effects on the body, though both can influence appetite regulation and gastric emptying. In clinical trials, neither medication lists menstrual disruption as a primary or very common side effect. However, both are associated with gradual weight management, and significant weight loss — whether from medication or lifestyle changes — can temporarily affect period regularity, particularly if it occurs quickly. Some women on Mounjaro report lighter or more irregular periods; others report no change. Individual experiences vary. If you're on a UK prescription for either medication following a clinical assessment, your prescriber should be informed of any changes to your menstrual cycle so they can assess whether they relate to the medication, the weight loss itself, or another factor.
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.
Individual Factors and Prescriber Guidance
Changes to your menstrual cycle on Mounjaro or Wegovy may reflect several factors: gradual weight management, hormonal changes, dose adjustments, or unrelated health factors. The NHS recommends losing no more than 0.5–1 kg per week for sustainable results, which is gentler on hormone balance than very rapid loss. If you experience significant period changes — such as missed periods, very heavy bleeding, or severe pain — contact your GP or the prescriber who issued your medication. This is not necessarily a reason to stop treatment, but it requires assessment. Both medications require a UK prescription following clinical evaluation, and ongoing monitoring includes discussing any side effects or changes to your health. Use our free BMI calculator to understand your starting point, and discuss any concerns about menstrual cycle changes before starting or adjusting your dose.
- 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 cause missed periods?
Mounjaro may indirectly affect your menstrual cycle through weight loss, which can influence hormone levels. Missed periods are not listed as a common side effect in clinical trials, but gradual weight management can temporarily disrupt cycle regularity. If you miss periods while on Mounjaro, contact your GP or prescriber for assessment — it may relate to the medication, the weight loss, or another factor.
Does Wegovy affect periods differently than Mounjaro?
Both are GLP-1-based medications (Wegovy is GLP-1 only, Mounjaro is GIP+GLP-1), so effects on menstrual cycle are likely similar. Individual experiences vary. Neither lists menstrual disruption as a common side effect, but weight loss itself can influence cycle regularity. Your prescriber can assess your individual response.
Is it safe to take Mounjaro if I'm trying to get pregnant?
No. Mounjaro is not recommended if you're pregnant or planning pregnancy. If you're taking Mounjaro and want to conceive, contact your GP or prescriber. Women taking oral contraceptives should use additional barrier protection for 4 weeks after starting Mounjaro and after each dose increase, as the medication may reduce absorption.
What should I do if my period changes on a GLP-1 injection?
Contact your GP or the prescriber who issued your medication. Changes may reflect weight loss, dose adjustment, or an unrelated health factor. Your prescriber can assess whether the medication needs adjusting or whether the change is expected and will stabilise over time.
Do I need to stop my weight loss injection if my cycle becomes irregular?
Not necessarily — but you must inform your prescriber. Menstrual changes don't automatically mean you should stop treatment. Your prescriber will assess whether the medication is suitable to continue, whether the dose needs adjusting, or whether to monitor further. Never stop a prescription medication without medical guidance.
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.