Antibiotics are one of the most commonly used drugs in veterinary medicine. Some of the adverse effects of antibiotics (eg, vomiting or diarrhea) are well-known and well-recognized, while others are uncommon or idiosyncratic. In this study, the effects of 4 commonly used antibiotics on primary (platelet number and function) and secondary hemostasis (processes required to form a stable secondary platelet plug) were evaluated. Doxycycline (10 mg/kg PO Q 12 H), amoxicillin (30 mg/kg PO Q 12 H), cephalexin (30 mg/kg PO Q 12 H), and enrofloxacin (20 mg/kg Q 12 H) were each administered for 7 days to healthy beagles. Pre- and posttreatment platelet counts, prothrombin time (PT), activated partial thromboplastin time (APTT), fibrinogen concentration, hematocrit, and platelet function were evaluated. No significant changes were noted in total white blood cell count, hematocrit, or platelet counts. There also was no significant change in primary hemostasis as evaluated by buccal mucosal bleeding time, closure time measured by a platelet function analyzer, or platelet aggregation. There was a general trend found for all drugs to cause a slightly prolonged PT and APTT and a decrease in fibrinogen concentration. For some drugs, these changes were significant. Enrofloxacin caused the most significant decrease in fibrinogen concentration. Cephalexin administration was associated with the most significant increase in PT and APTT. Amoxicillin caused a significant increase in PT. Although these changes were significant, none of them appeared to be clinically relevant to healthy dogs. The authors note that the duration of drug administration was shorter than might be used in an animal with a medical condition. Although these changes were clinically irrelevant to these dogs, they may be important in dogs with preexisting disease. Study funded by the Ontario Veterinary Pet Trust Companion Animal Research Fund

COMMENTARY: The take-home message is that some antibiotics can cause a prolongation of the results of some hemostatic tests. In these healthy dogs, the increase was statistically, but not clinically, significant (ie, it did not produce clinical bleeding). It is important to note that a 1- to 2-second increase, compared with a 16-second increase, is considered statistically significant. You don't have to read the article to file away this "factoid"-just keep in mind that there are many medications that can alter primary or secondary hemostasis. When a patient is being treated with several medications, side effects may combine to create a clinically important syndrome.

Effects of doxycyline, amoxicillin, cephalexin, and enrofloxacin on hemostasis in healthy dogs. Webb JA, Allen DG, Abrams-Ogg AC, Gentry PA. AM J VET RES 67:569-576, 2006.