Adverse reactions to medications create an important health problem and a nursing professional must assume his/her role in detecting them early on, in identifying them, in educating the public about them, in treating them, and finally in serving notice of them. In order to ease the identification of adverse effects which patients may present, the authors provide general information about adverse reactions and factors related to a greater risk occurrence of those reactions. Based on a practical case, the authors respond to five questions on features having casual relationship between a pharmaceutical product and an adverse reaction by means of which it is possible to define the plausibility of suspected adverse effects. By using this simple reasoning method, nurses will be capable of having a grounded criteria in relation to adverse reactions which a patient can present and thus contribute to decision making by the professional team as to what is the procedure to follow.