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
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How Do Periods Change on Mounjaro and Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) can affect your menstrual cycle, though not everyone experiences changes. gradual weight management itself, rather than the medication specifically, is often the primary driver of period changes. Understanding what to expect and how these two prescription weight loss injections compare can help you make an informed decision alongside your GP or prescriber. Individual experiences vary significantly, and any concerns should be discussed with your healthcare provider.
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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 Medication Mechanisms Differ and Impact Cycles
Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates only the GLP-1 pathway. This mechanistic difference doesn't directly alter how either medication affects menstrual cycles — both work primarily through appetite suppression and weight loss. The key driver of period changes is the rate and amount of weight loss itself. gradual weight management can temporarily disrupt hormone production, particularly if calorie intake drops significantly. Neither Mounjaro nor Wegovy is specifically known to cause period irregularities beyond what occurs with significant weight loss in general. Clinical trial data from SURMOUNT-1 (Mounjaro) and STEP-1 (Wegovy) did not highlight menstrual changes as a common side effect, though participants reported weight loss averaged 15–21% on Mounjaro and approximately 15% on Wegovy. If you experience prolonged period changes, discuss this with your GP — it may indicate gradual weight management, nutritional deficiency, or an underlying condition unrelated to the medication.
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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.
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Menstrual Changes and Weight Loss — What the Evidence Shows
Weight loss of any kind can affect your menstrual cycle because hormonal balance depends on adequate body fat and nutrition. When you lose weight rapidly, oestrogen and other reproductive hormones may fluctuate, potentially causing irregular periods, lighter or heavier bleeding, or temporary amenorrhoea (missed periods). Both Mounjaro and Wegovy produce significant weight loss — on average 15–21% body weight on Mounjaro and around 15% on Wegovy — so menstrual changes are possible with either medication. However, sustainable weight loss alongside a balanced diet and adequate calorie intake (not extreme restriction) minimises these risks. Use our free GLP-1 weight loss calculator to estimate realistic weight loss progression, which tends to be more gradual and sustainable than crash dieting. Your GP can monitor your cycle alongside your weight management plan.
- 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 stop your period?
Mounjaro itself doesn't directly stop periods, but gradual weight management can. If you lose weight very quickly (especially below a healthy BMI for your height), your body may temporarily pause menstruation due to hormone changes. This is reversible — periods typically return when weight stabilises or increases slightly. Always discuss prolonged missed periods with your GP.
Is it normal to have irregular periods on Wegovy?
Yes, some users report irregular periods or lighter bleeding on Wegovy, usually linked to weight loss rather than the medication directly. If you're losing weight steadily (0.5–1kg per week, which is healthy), minor cycle changes may occur. Severe or prolonged irregularities should be reviewed by your GP to rule out other causes.
Which is better for menstrual health — Mounjaro or Wegovy?
Neither medication has a clear advantage for menstrual health. Both cause similar levels of weight loss, which is the main factor affecting your cycle. Your prescriber will choose based on your overall health, BMI, and medical history — not specifically on cycle protection. Sustainable weight loss with adequate nutrition helps both.
Will my period return to normal after stopping Mounjaro or Wegovy?
Yes, in most cases. Once your weight stabilises and you're no longer losing rapidly, your hormone levels typically re-regulate and periods return to their baseline pattern. This may take a few months. If periods don't return after 3–6 months of weight stability, consult your GP.
Should I avoid these injections if I have irregular periods already?
Not necessarily. Both Mounjaro and Wegovy are prescribed to people with pre-existing period irregularities. Discuss your cycle history with your prescriber during your clinical assessment — they'll assess whether the medication is safe for you and may monitor your cycle during treatment. Many users find that weight loss actually improves hormone balance long-term.
Ready to take control of your weight loss journey?
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