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Background:  Inflammation is present in several conditions associated with risk of venous thromboembolism. The gut microbiome might be a source of systemic inflammation and activation of coagulation, by translocation of lipopolysaccharides from gram-negative bacteria to the systemic circulation., Objective:  To investigate whether a vancomycin-induced shift of the gut microbiome in a gram-negative direction influences systemic inflammation and plasma factor (F) VIII procoagulant activity (FVIII:C)., Methods and Results:  We performed a randomized controlled trial including 43 healthy volunteers aged 19 to 37 years. Twenty-one were randomized to 7 days of oral vancomycin intake and 22 served as controls. Feces and blood were sampled at baseline, the day after the end of intervention, and 3 weeks after intervention. Gut microbiome composition was assessed by amplicon sequencing., Fviii: C was measured using an activated partial thromboplastin time-based assay, cytokines were measured using multiplex technology, complement activation was measured using the enzyme-linked immunosorbent assay, and high-sensitivity C-reactive protein (CRP) was measured by an immunoturbidimetric assay. Vancomycin intake reduced gut microbiome diversity and increased the abundance of gram-negative bacteria. Change in FVIII:C in the intervention group was +4 IU/dL versus -6 IU/dL ( p  = 0.01) in the control group. A similar change was observed for log-transformed CRP (+0.21 mg/dL vs. -0.25 mg/dL, p  = 0.04). The cytokines and complement activation markers remained similar in the two groups., Conclusion:  The found slight increases in FVIII:C and CRP levels might support the hypothesis that a vancomycin-induced gram-negative shift in the gut microbiome could induce increased systemic inflammation and thereby a procoagulant state., Competing Interests: None declared., (Thieme. All rights reserved.)