Medical reasoning, the method of solving clinical problems, is the foundation of all the decisions physicians make, aiming to understand the illness and arrive at the appropriate therapeutic decisions. Interaction among different physicians and exchange of opinions may often lead to disagreement with respect to the diagnostic or treatment priorities. The quality of the arguments presented comes in focus, making it necessary to be aware of and familiar with the logical fallacies, i.e. flawed ways of reasoning. Some of the more commonly encountered types of fallacies are described, along with examples to help clarify their substance. Logical fallacies can have a toxic effect, leading to improper medical decisions. Safeguarding medical reasoning is of paramount importance; adopting a critical method, actively seeking to identify erroneous arguments by asking appropriate questions is presented. Awareness of the presence and the features of flawed reasoning is a profoundly important skill for all physicians, an integral part of our ability to process clinical information efficiently and correctly.