Pharmacology principles provide key understanding that underpins the clinical and research roles of nuclear medicine practitioners. The scope of practice of the nuclear medicine technologist demands knowledge and understanding of indications, contraindications, warnings, precautions, proper use, drug interactions, and adverse reactions for each medication to be used. This article is part of a series that aims to enhance understanding of pharmacologic principles relevant to nuclear medicine. This article will build on the introductory concepts, terminology, and principles of pharmacology explored in the first 2 articles in the series. Specifically, this article will focus on the pharmacologic principles and adverse reactions associated with iodinated and gadolinium contrast media used in CT and MRI, respectively.