Aspirin, even in small doses, has a profound and relatively long-lasting effect on platelet function and hemostasis. This usually produces few clinical problems. However, if a patient has an underlying hemostatic defect, undergoes surgery or sustains an injury, or is a newborn, severe hemorrhage is a potential risk. Aspirin is contraindicated in these clinical contexts. In contrast, acetaminophen has no effect on the hemostatic mechanism unless massive overdose results in hepatic necrosis with depression of synthesis of coagulation factors. Acetaminophen can be used when indicated in clinical situations where the use of aspirin may be potentially dangerous.