Background: Menstrual tampons are widely used in western countries. Indirect evidence suggests that tampon misuse could be associated with an increased risk of menstrual toxic shock syndrome (MTSS). The aim of this study was to determine what characteristics of tampon use are associated with increased risk of menstrual toxic shock syndrome (MTSS). Methods: A nationwide, case-control study in France, was conducted with women that use tampons with MTSS diagnoses according to the CDC diagnostic criteria (n = 55, from January 2011, to December 2017) and a control group of women with no MTSS history (n = 126, from February to December 2017). Information regarding tampon use during a 6-month period was collected. Associations between tampon use and MTSS were assessed using logistic regression models stratified by residential area. Findings: Compared to controls, women diagnosed with MTSS more frequently reported maximum tampon wear of >6 h (62% vs. 41%; P = 0.02), overnight tampon use (77% vs. 54%; P = 0.006), and neither read nor followed tampon instructions in case of reading (65% vs. 42%; P = 0.006). In univariate analysis, MTSS risk was two-fold higher with tampon use for >6 consecutive hours (odds ratio, 2.3 [95% CI, 1.2À4.5]), and three-fold higher with tampon use during sleep for >8 h (odds ratio, 3.2 [95% CI, 1.4À7.7]). In multivariate logistic regression analysis, only maximum tampon use for >6 h (odds ratio, 2.03 [95% CI, 1.04À3.98]), and neither read nor followed the tampon instructions in case of reading (odds ratio, 2.25 [95% CI, 1.15À4.39]) were independently associated with MTSS. Interpretation: Our study suggests that the risk of MTSS was associated with using tampons for more than 6 h, overnight tampon use during sleep, and neither read nor followed tampon insertion instructions in case of reading. Funding: LABEX ECOFECT (ANR-11-LABX-0048) of Universit e de Lyon within the programme "Investissements d'Avenir" (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR).