Spanish Royal Legislative Decree 1/2015 of July 24 considered radiopharmaceuticals to be drugs subject to current legislation on this matter. After Radiopharmaceuticals are prepared in Radiopharmacy Departments, they are administered to patients to observe alterations or their abnormal distribution in the body depending on the pathology. Unlike conventional drugs, radiopharmaceuticals rarely cause adverse reactions: however, when they do occur, they often cause alarm both to the patient and health care staff. They are typically administered to patients only once or a limited number of times. Most adverse reactions to radiopharmaceuticals are mild and require minimal or no treatment. Such reactions, although rare, can occur, and include sensitivity reactions with systemic symptoms. In general, the most common adverse reactions to radiopharmaceuticals are the following: nausea, dyspnea, bronchospasm, decrease in blood pressure, itching, flushing, hives, chills, cough, bradycardia, muscle cramps, and dizziness. The incidence of reported adverse reactions to radiopharmaceuticals is low. The objective of this review is to describe the most common published adverse reactions to diagnostic or therapeutic radiopharmaceuticals and their characteristics.