Can I Start Mounjaro on My Period? (October 2026)

01 · The short answer

Is it fine to start Mounjaro while bleeding?

Yes, it is fine to start while bleeding. Mounjaro is a once-weekly injection licensed for weight management in adults, and the product information gives no instruction to time the first dose to your menstrual cycle.¹¹³ It works by activating GIP and GLP-1 receptors to help regulate appetite and slow how quickly the stomach empties.¹

The practical question is comfort. If your period brings cramps, tiredness or an upset stomach, you may prefer to begin a few days later so you can tell what is causing what. That is a personal choice, not a medical requirement.

02 · Your decision

What should you do next?

Complete the free consultation and tell the prescriber about your cycle, any contraception and any plans for pregnancy. A GPhC-registered prescriber reviews every order. If you are ready, the Mounjaro pen starts with the 2.5mg dose, and the weight loss clinic explains how our service works.

03 · Overlapping symptoms

Side effects that can overlap with period symptoms

Early side effects of Mounjaro are mainly digestive: the most common, affecting more than 1 in 10 people, are nausea, diarrhoea, vomiting, constipation and abdominal discomfort, and they are most likely when starting or moving up a dose.¹³ Period symptoms can be similar, so a few days of nausea or low energy may be hard to separate from your cycle. If a symptom is severe or does not settle, contact your prescriber. If you have severe, persistent stomach pain, get medical help straight away.

Our guide on whether you can inject during a period and the page on whether you can take it during a period go into this further.

04 · Does it change periods?

Can Mounjaro change or bring on a period?

Changes to your cycle are not on the list of side effects in the UK patient leaflet.¹³ Cycles can vary for many reasons, including changes in diet and body weight. If you notice a new pattern, such as a missed, unusually heavy or unexpected bleed, discuss it with your prescriber and report it through the Yellow Card scheme.¹⁹

We cover this in more detail in can Mounjaro bring on a period and period on Mounjaro: how does it work.

05 · Contraception

Contraception and Mounjaro

Mounjaro can reduce absorption of oral contraceptives because it slows how quickly the stomach empties.²⁷ The SmPC advises switching to a non-oral method, or adding a barrier method (such as condoms), for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.¹ Tell your prescriber which contraception you use so they can advise what is suitable.

06 · Dose and timing

Choosing an injection day

Mounjaro is taken once weekly, so choose a day you can repeat. Follow your prescriber's titration plan and the pen's instructions rather than moving doses around your cycle. Our page on what dose Mounjaro starts on explains the starting strength, and if you have paused treatment, read about whether you can stop and start Mounjaro. Treatment is used alongside a reduced-calorie diet and more physical activity, and individual results vary.¹

07 · Serious side effects

Serious side effects: when to get help straight away

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for: severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion) — your prescriber may need to adjust those medicines.¹