Drug allergy describes clinical adverse reactions that are proved or presumed to be immunologically based. Allergic drug reactions do not resemble pharmacologic actions of the incriminated drug and may occur at fractions of what would be the therapeutic dosage. Allergic drug reactions are unpredictable; nevertheless, there is increased risk of drug hypersensitivity in (1) patients with cystic fibrosis who receive antibiotics; (2) patients with human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) who receive trimethoprim/sulfamethoxazole of if HLA-B*5701(+) and receive the antiretroviral agent, abacavir; (3) other genetically susceptible populations such as Han-Chinese who are HLA-B*1502(+) who develop Stevens-Johnson syndrome and toxic epidermal necrolysis from carbamazepine or if HLA-B*5801(+) are at increased risk for such reactions from allopurinol; and (4) patients with a history of previous compatible allergic reaction to the same medication, similar class, or potentially unrelated medication. Specific patient groups at higher risk for drug allergy include those with Ebstein-Barr virus infection, chronic lymphatic leukemia, HIV/AIDS, cystic fibrosis, patients with seizures being treated with antiepileptic medications, and patients with asthma (especially severe asthma) who are at increased risk of anaphylaxis from any cause including drugs compared with patients without asthma. In patients with a history of penicillin allergy, skin testing helps clarify the current level of risk for anaphylaxis by using the major (penicilloyl-polylysine) and minor penicillin determinants where sensitivity is 99%. If penicilloyl-polylysine and penicillin G are used for skin testing, the sensitivity is ∼85%. When skin tests are negative, graded challenges are performed to administer optimal or truly essential antibiotics.