Following minor changes of excipients of Levothyrox®, the French Pharmacovigilance Database was overwhelmed by patients' spontaneous reports of adverse drug reactions associated with the new formula. After noticing that most of these reports differed from those related to other drugs, we aimed to characterize their features and compared them with spontaneous reports associated with other chronic treatments as comparators. We randomly sampled patient reports associated with either Levothyrox® new formula (n=200) or with comparator drugs (n=200) from March 2017 till March 2018 from the national pharmacovigilance database. We evaluated the number of incriminated drugs and adverse drug reactions per report, and verified whether they were "expected" or not according to the Summary of Product Characteristics. Levothyrox® associated reports included, on average, more adverse drug reactions (8±4) than comparators (2±2, p<0.01), and mentioned mostly 1 drug (98.5% of reports) whereas comparators mentioned 2 at least (p<0.001). The quantitative distribution of adverse drug reactions per report differed quite significantly, appearing almost Gaussian for Levothyrox® whereas Poisson-like for comparators (p<0.0001). Age did not differ significantly in the two groups (54.2 versus 49.7, p>0.05), but female predominated in Levothyrox® group (94.5%) as compared with comparators (60.8%, p<0.001). A mere third of the Levothyrox® associated adverse drug reactions were deemed "expected", versus two-third for comparators (p<0.001). The pattern of spontaneous reports associated with Levothyrox®, whether fueled by media or influenced by social networks, appears atypical, as compared with that of comparators. Such reports, by their abundance may impair the automatic detection of relevant concomitant signals.< Réduire