Febrile non-hemolytic and allergic reactions are the most common transfusion reactions, but usually do not cause significant morbidity. In an attempt to prevent these reactions, US physicians prescribe acetaminophen or diphenhydramine premedication before more than 50% of blood component transfusions. Acetaminophen and diphenhydramine are effective therapies for fever and allergy respectively, so their use in transfusion has some biologic rationale. However, these medications also have potential toxicity, particularly in ill patients, and in the studies performed to date, they have failed to prevent transfusion reactions. Whether the benefits of routine prophylaxis with acetaminophen and diphenhydramine outweigh their risks and cost requires re-examination, particularly in light of the low reaction rates reported at many institutions even when pre-medication is not prescribed.The most common acute adverse reactions to blood-component transfusions, febrile nonhemolytic transfusion reactions (FNHTRs) and allergic reactions, are fortunately among the least harmful. Blood recipients with FNHTRs experience fever (often defined as a temperature rise ≥ 1°C above baseline) and/or rigors within 3 hours of transfusion. Allergic reactions are most often associated with the development of urticaria or other rash, pruritus, wheezing, or angioedema within several hours of transfusion. 1-8 These reactions are temporally limited, self-resolving, and generally pose little risk of causing lasting harm. Mild reactions often consist of a limited increase of temperature without other symptoms or a localized urticarial exanthem. In moderate and severe reactions, rigors and fever may be severe with rapid onset and associated with other symptoms, and urticaria may be extensive and painful or include respiratory or other systemic symptoms.How an incipient urticarial or FNHTR will progress cannot be predicted on the basis of its initial presentation. Further, it is often not possible to distinguish the symptoms of mild reactions from early symptoms of other more consequential problems such as sepsis, hemolytic reactions, or anaphylaxis. Therefore, the earliest indication of a reaction mandates discontinuation of a transfusion and, except for the mildest urticarial reactions, its termination. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errorsmaybe discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Increased clinical and laboratory monitoring is necessary, and infusion of replacement blood may be required. Blood replacement is associated with additional transfusion-associated risks and may deplete low blood inventories. The development of repeated rea...