Background In the year 2020, the coronavirus pandemic invaded the world. Since then, specialized companies began to compete, producing many vaccines. Coronavirus vaccines have different adverse events. Menstrual disorders have been noticed as a common complaint post-vaccination. Aim Our study fills an important gap by evaluating the relationship between coronavirus vaccines and menstrual disorders. Methods This is a cross-sectional study between 20 September 2021, and 1 October 2021, using an online survey. The questionnaire consisted of 36 questions divided into 4 sections: demographics, COVID-19 exposure and vaccination, hormonal background, and details about the menstrual cycle. Sample t -test, ANOVA test, chi-square, and McNemar test were used in bivariate analysis. Results This study includes 505 Lebanese adult women vaccinated against COVID-19. After vaccination, the number of women having heavy bleeding or light bleeding increased ( p = 0.02 and p < 0.001, respectively). The number of women having regular cycles decreased after taking the vaccine ( p < 0.001). Irregularity in the cycle post-vaccination was associated with worse PMS symptoms ( p = 0.036). Women using hormonal contraception method or using any hormonal therapy had higher menstrual irregularity rates ( p = 0.002 and p = 0.043, respectively). Concerning vaccine adverse events, those who had headaches had a higher rate of irregularity ( p = 0.041). Those having PCOS, osteoporosis, or blood coagulation disorders had higher irregularity rate ( p < 0.001 and p = 0.005, respectively). Conclusion Vaccine adverse events may include specific menstrual irregularities. Moreover, some hormonal medications and diseases are associated with the alteration of the menstrual cycle. This study helps in predicting vaccines’ menstrual adverse events, especially in a specific population prone to menstrual disorders.

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