The objective of this study was to assess the impact of tigecycline treatment on coagulation parameters, specifically fibrinogen, in patients with severe infections. We examined 20 cases of tigecycline-treated patients with severe infections, including hospitalacquired pneumonia, complicated intra-abdominal infections, complicated skin and soft tissue infections, and bloodstream infections. We monitored the relative markers of coagulation and renal and liver function before, during, and after treatment. Fibrinogen (FIB) levels decreased significantly after the use of tigecycline and normalized after the cessation of treatment. FIB levels significantly decreased in the patients treated with the recommended dose or a higher treatment dose. The FIB levels decreased more in the higher-treatment-dose group. There was no difference in the decrease in FIB levels or the FIB level recovery by age. Prothrombin time (PT), activated partial thromboplastin time (APTT), and thrombin time (TT) were prolonged after tigecycline use. The TT decreased after the cessation of treatment, and the PT and APTT also decreased but not to a significant level. There was no change in platelet, alanine aminotransferase (ALT), or creatinine (Cr) levels associated with treatment. The use of tigecycline was associated with decreased FIB levels, which returned to normal after the cessation of treatment. A highdose treatment group showed greater decreases in FIB levels than did patients treated with the recommended dose. The decline in FIB was not related to patient age. The use of tigecycline was associated with prolonged PT, APTT, and TT.T igecycline is a novel broad-spectrum intravenous antibiotic that inhibits bacterial protein synthesis. Tigecycline functions by binding to the 30S ribosomal subunit and preventing aminoacylated tRNA molecules from entering the ribosomal A site. Tigecycline contains a glycylamino group substituted for the 9-minocycline group, a substitution rarely seen in any natural or semisynthetic tetracycline compound, which gives tigecycline unique microbiological properties. Tigecycline is not affected by the two major bacterial tetracycline resistance mechanisms, efflux and ribosomal protection mechanisms (1). Tigecycline has a very broad spectrum of antibacterial activity, with Grampositive cocci (including methicillin-resistant Staphylococcus aureus, [MRSA] and vancomycin-resistant enterococci [VRE]) (2), Gram-negative bacilli (including Acinetobacter spp. and Stenotrophomonas maltophilia) (3), and anaerobic bacteria (4) having relatively high sensitivities. Kadoyama et al. (5) summarized a total of 248 tigecycline-related adverse reactions, the most common being nausea, vomiting, elevated levels of alanine aminotransferase, bilirubin, alkaline phosphatase, and aspartate aminotransferase, and hepatic dysfunction. In a number of studies, patients showed good tolerance of tigecycline, indicating that it is safe (6-7). However, there were two case reports that tigecycline can induce decreases in fibrinogen (FIB) levels (...