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 Menstrual Cycle?
Mounjaro (tirzepatide) is a prescription-only weight management injection that may affect your menstrual cycle in some cases. Some women using Mounjaro report changes to their periods, including irregular cycles, heavier or lighter bleeding, or missed periods. These changes can occur because gradual weight management and hormonal shifts may temporarily disrupt your cycle. However, not everyone experiences period changes, and individual responses vary significantly. If you notice unusual changes to your menstrual cycle whilst taking Mounjaro, it's important to speak with your GP or prescriber for guidance.
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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 Period
Mounjaro works as a GIP and GLP-1 receptor agonist, a dual-mechanism injection that may help with weight management through appetite regulation and satiety. During the dose titration phase, when your body is adjusting to tirzepatide, hormonal changes can influence your menstrual cycle. gradual weight management itself—which Mounjaro may support—can temporarily alter oestrogen levels and disrupt menstrual regularity, particularly if loss exceeds 1 kg per week over an extended period.
Common menstrual changes reported by women using Mounjaro include:
- Irregular or delayed periods
- Lighter or heavier menstrual bleeding
- Temporary amenorrhoea (missed periods)
- Changes to cycle length or duration
- Spotting between periods
These effects are often temporary and may stabilise as your body adjusts. However, severe or persistent changes should be reported to your GP or prescriber immediately.
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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.
When to Speak to Your GP or Prescriber
You should contact your GP or prescriber if you experience severe abdominal pain, heavy vaginal bleeding, or periods that stop for more than three months. Whilst minor period irregularities are often expected during significant weight loss, your healthcare provider needs to rule out other causes and ensure Mounjaro remains appropriate for you.
If you're considering Mounjaro UK prescription, discuss any history of menstrual disorders, hormonal imbalances, or concerns about cycle changes during your clinical assessment. Your prescriber can monitor your progress using MyWeightCheck's free GLP-1 weight loss calculator to ensure weight loss is occurring at a safe, sustainable rate. Remember that tirzepatide requires ongoing medical supervision, and your prescriber may adjust your dose or offer additional support if menstrual changes become problematic.
- 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 permanent changes to my menstrual cycle?
Most menstrual changes during Mounjaro treatment are temporary and relate to gradual weight management and hormonal adjustment rather than the medication itself. Once your weight stabilises or you reach maintenance, cycles typically return to normal. However, if changes persist beyond six months, your GP should investigate other potential causes.
Will my period come back if I stop taking Mounjaro?
In most cases, yes. If menstrual irregularities are linked to Mounjaro and weight loss, your cycle typically normalises within a few months of stopping treatment as your weight and hormones stabilise. Always discuss stopping medication with your prescriber rather than stopping abruptly.
Is Mounjaro safe to use if I have endometriosis or PCOS?
Women with endometriosis or PCOS (polycystic ovary syndrome) should discuss their condition with their prescriber during the clinical assessment. Whilst Mounjaro itself is not contraindicated, gradual weight management may affect these conditions differently. Your GP can advise whether Mounjaro is appropriate for your individual situation.
How does Mounjaro compare to Wegovy regarding menstrual effects?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) may cause temporary menstrual changes due to weight loss and hormonal shifts. Mounjaro uses a dual GIP and GLP-1 mechanism, whilst Wegovy uses GLP-1 only. Individual experiences vary, but menstrual effects are linked more to the pace of weight loss than to the specific medication.
Should I use additional contraception while taking Mounjaro?
Yes. If you take oral contraceptives, your prescriber will advise using a barrier method or switching contraception for four weeks after starting Mounjaro and four weeks after each dose increase. Mounjaro may reduce oral medication absorption, including the contraceptive pill, so additional protection is essential.
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.