Where cycles come up in the Mounjaro® documents
No menstrual row appears in the US trial tables, the EU frequency table or the Canadian tables [1][3][4]. The relevant information sits elsewhere.
| Topic | US label | EU information | Canada |
|---|---|---|---|
| Menstrual side effect | Not listed [1] | Not listed [3] | Not listed [4] |
| Animal fertility study | Longer resting phase, fewer corpora lutea in female rats [1] | No direct harm to fertility [3] | Same rat findings [4] |
| Human fertility | Not stated [1] | Unknown [3] | Not stated [4] |
| Pregnancy | Only if benefit justifies fetal risk [1] | Not recommended; stop 1 month before planned pregnancy [3] | Contraindicated; stop 1 month before [4] |
| Birth-control pills | Switch or add a barrier for 4 weeks after start and each increase [1] | Exposure drop not considered clinically relevant [3] | Same advice as US [4] |
The rat finding, in plain terms
In fertility studies, female rats given tirzepatide more often had a prolonged diestrus, the resting phase of the rat cycle, and fewer corpora lutea, the structures left after an egg is released. This led to fewer implantation sites and viable embryos, and it happened at all dose levels tested. Male rats showed no effects on sperm, mating or fertility [1][4].
The US label considers these effects secondary to tirzepatide’s effects on food intake and body weight [1], and Europe concludes that animal studies did not indicate direct harm to fertility [3]. Rat cycles are not human menstrual cycles, and no human study has measured cycle length on tirzepatide.
What human studies report
A 2026 analysis of US FDA spontaneous reports for tirzepatide, more than 67,000 cases, listed menstrual disorder and bleeding after menopause among new statistical signals and suggested a possible effect on sex hormones [5]. Spontaneous reports are written by patients and professionals without a comparison group, so they show what people notice, not how often it happens or whether the medicine is responsible.
Most trial evidence comes from polycystic ovary syndrome, a hormonal condition that often causes irregular periods. In a 16-week randomised, open-label trial in China, 60 women with PCOS and overweight or obesity took metformin alone or metformin plus tirzepatide. The combination group lost more weight and had higher rates of menstrual cycle recovery and, after treatment, of pregnancy; barrier contraception was required during part of the study [6]. A 2026 meta-analysis of randomised GLP-1 trials in PCOS, searched up to September 2024, found the evidence on menstrual regularity too uncertain to draw conclusions [7].
Why a cycle might shift on treatment
- Supported by the label: tirzepatide reduces food intake and body weight, and the rat cycle changes were tied to exactly that [1].
- General medicine: MedlinePlus lists obesity and sudden large weight loss among things that make missed periods more likely, and severe weight loss or large weight change among causes of abnormal bleeding [9][10]. Weight change in either direction can matter.
- Possible and unmeasured: if periods were irregular because of PCOS, weight loss might make them more regular; the PCOS trial points that way, but it is small and short [6].
- Not a direct effect: birth-control pills may work less well around the start and dose increases, according to the US and Canadian labels [1][4]. The details are on our GLP-1 and oral contraceptives page; the point here is that an unexpected pregnancy is one possible cause of a missed period.
Other causes of period changes
MedlinePlus names pregnancy as the most common cause of missed periods, along with breastfeeding and menopause. Others include hormonal contraception, intense exercise, severe stress, an overactive thyroid, pituitary problems, PCOS, reduced ovarian function and some medicines [9].
Abnormal uterine bleeding, meaning bleeding that is too frequent, too heavy, too long or between periods, is often due to a hormone imbalance, especially in teenagers and people approaching menopause. Fibroids, polyps, infections, bleeding disorders, hormonal birth control and, less often, cancers of the reproductive organs are other causes [10].
Mounjaro® is approved for children 10 years and older with type 2 diabetes, and 61% of the children in its trial were girls [1]. Periods often take a few years to become regular after they start [11]. A 2026 commentary on GLP-1 medicines in adolescents calls for contraceptive counselling and notes that reproductive outcomes after teenage use are not known [8].
What to do if your cycle changes
The Mounjaro® documents do not address period changes, so the steps below come from MedlinePlus and the labels’ pregnancy sections.
- If a period is late and pregnancy is possible, take a pregnancy test and tell your prescriber if it is positive [1][2].
- Keep a record of your period dates so you know your usual pattern and can spot changes [11].
- See your provider if you miss more than one period [9].
- Contact your provider about unusual vaginal bleeding, including bleeding between periods or after menopause [10].
- If you plan a pregnancy, talk with your prescriber in advance; Europe and Canada advise stopping tirzepatide at least a month beforehand [3][4].
Mounjaro® is a registered trademark of Eli Lilly and Company. PepLabo is not affiliated with or endorsed by Eli Lilly and Company.
When to get help
| Sign | Who to contact | How fast | Source |
|---|---|---|---|
| Very heavy bleeding, such as soaking through a pad or tampon every hour for 2 to 3 hours in a row | Contact your provider promptly | Today | MedlinePlus [10] |
| A positive pregnancy test while using Mounjaro® | Tell your prescriber | Today | Medication Guide [2]; Canadian patient information [4] |
| Bleeding between periods or any bleeding after menopause | Your provider | Today | MedlinePlus [10] |
| More than one missed period, with a negative pregnancy test | Your primary or women’s health provider | At your next appointment | MedlinePlus [9] |
| Planning a pregnancy | Discuss timing with your prescriber | At your next appointment | EU information [3]; Canadian monograph [4] |
