<p>Purpose: Thrombocytopenia is a major adverse effect of linezolid. Although proton pump inhibitors (PPIs) are frequently used in patients with linezolid, the impact of PPIs on linezolid-induced thrombocytopenia remains to be explored. Here, we examined the drug interaction between PPIs and linezolid by retrospective cohort study, Food and Drug Administration Adverse Event Reporting System (FAERS) database analysis, and cell lines.</p><p>Methods:The retrospective cohort study was conducted in 150 hospitalized patients with linezolid. Thrombocytopenia was defined as a reduction in platelet counts ＜ 100 (×10⁹/L) or a reduction more than 30% from the baseline value. Uptake study of linezolid and major metabolites (PNU-142300 and PNU-142586) were examined in HEK-hOAT3 and HEK-pBK cells. The effect of linezolid and metabolites on the cell viability were determined using megakaryoblast cell line MEG-01 cells.</p><p>Results:In a retrospective study, multivariate analysis revealed that coadministration of lansoprazole (LPZ), but not other PPIs, was an independent risk factor significantly contributing to the thrombocytopenia (odds ratio: 2.33, p = 0.034). Moreover, FAERS database analyses revealed that the reporting ratio of thrombocytopenia of LPZ was 1.64 (95% CI: 1.25-2.16). Uptake of PNU-142586, but not linezolid and PNU-142300, was significantly higher in HEK-hOAT3 cells than that in HEK-pBK cells. The inhibitory effect of LPZ was much greater than those of other PPIs and the apparent IC₅₀ value of LPZ against hOAT3-mediated transport of PNU-142586 was 0.59 ± 0.22 μM. Moreover, the cell viability in MEG-01 cells was decreased in a concentration dependent manner of PNU-142586.</p><p>Conclusion: These findings suggested that concomitant LPZ exacerbates linezolid-induced thrombocytopenia, at least in part, by inhibiting hOAT3-mediated transport of PNU-142586.</p>