This review focuses on the epidemiology and different risk factors related to the development of hypersensitivity reactions to antibiotics, with a focus on betalactams and fluoroquinolones, the compounds most frequently involved in these reactions, due to their high level of consumption. The true prevalence of allergic drug reactions is unknown and the corresponding morbidity, mortality and associated economic costs are often underestimated. It is reported that multiple risk factors, related to both the drug and the patient, can modify the clinical expression of immune-mediated drug reactions. These include the chemical properties, molecular weight and administration route of the drug and the age, gender, concomitant diseases and genetic factors of the patient.