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) is a prescription-only weight loss injection that may affect menstrual cycles in some women, though period changes are not listed as a direct side effect. Weight loss itself, rapid changes in body composition, and the dual GIP/GLP-1 mechanism may influence hormone levels and menstruation. If you're taking Mounjaro and notice changes to your period, it's important to discuss this with your GP, as individual experiences vary significantly and other factors may be involved.
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 May Influence Menstrual Cycles
Mounjaro works as a dual GIP and GLP-1 receptor agonist, activating hormonal pathways in the body. While the medication itself is not reported to directly alter hormone levels that control menstruation, several related factors may influence your cycle:
- gradual weight management — significant body composition changes can affect oestrogen and progesterone levels, potentially causing period irregularities, lighter periods, or temporary cessation (amenorrhoea)
- Metabolic changes — improved insulin sensitivity and blood sugar control may shift hormone balance
- Nutritional factors — reduced food intake on Mounjaro may lead to deficiencies in iron, zinc, or B vitamins, which support menstrual health
- Stress response — weight loss medication and lifestyle changes can trigger stress hormones that temporarily affect your cycle
These effects are typically temporary and often resolve once weight stabilises or nutritional intake is balanced.
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.
What You Should Know About Mounjaro and Contraception
Important contraceptive information: If you're taking oral contraceptives (the pill), Mounjaro may reduce how effectively they are absorbed, potentially lowering contraceptive protection. The MHRA prescribing guidance recommends adding a barrier contraceptive method (condoms) or switching to a non-oral contraceptive for 4 weeks after starting Mounjaro AND 4 weeks after each dose increase.
This is because Mounjaro slows gastric emptying—the rate at which food and medicines leave your stomach—which can affect how your body absorbs oral medications. If you're planning pregnancy, Mounjaro is not recommended, as tirzepatide has not been studied in pregnancy. Always inform your prescriber about all contraceptive methods before starting treatment.
- 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 period irregularity a listed side effect of Mounjaro?
Period changes are not formally listed as a common or uncommon side effect in the product information. However, women have reported menstrual irregularities, lighter periods, or missed periods during Mounjaro treatment. This is likely related to gradual weight management rather than the medication directly. Report any significant changes to your GP.
Will my period return to normal after stopping Mounjaro?
In most cases, menstrual cycles normalise once weight stabilises and Mounjaro is discontinued. However, individual timelines vary. If your period has not returned to normal 2–3 months after stopping treatment, contact your GP to rule out other causes.
Can Mounjaro interact with period pain medication?
Common period pain relievers like ibuprofen and paracetamol are not known to interact with Mounjaro. However, because Mounjaro slows stomach emptying, absorption of all oral medications may be slightly delayed. Take painkillers as directed and discuss any concerns with your pharmacist.
Should I stop Mounjaro if my period stops?
No—do not stop Mounjaro without guidance from your prescriber. Amenorrhoea (missing periods) during gradual weight management is often temporary. Contact your GP to discuss your symptoms. They may want to rule out other causes or adjust your treatment plan based on your individual health.
Does Mounjaro affect fertility or ovulation?
There is limited research on tirzepatide's effects on ovulation and fertility. Significant weight loss can improve fertility in women with obesity-related irregular cycles. If you're trying to conceive, discuss this with your GP before starting Mounjaro, as the medication is not recommended in pregnancy.
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.