The purpose of this book is to provide guidance on issues that can be confused in clinical practice such as allergies, intolerance, and poisoning to common substances like alcohol, certain foods (meats, shellfish, and vegetables), medications, metals, and vitamins, among others. Our experience in the study of allergic diseases and toxicology at the clinical and basic level has shown that there is some confusion and difficulty to understand the underlying mechanisms in the development of allergic responses and intolerance, for example, to shellfish. This often delays or may mislead the diagnosis and management of both clinical entities. For this reason, this book offers a structure that allows, first, to understand the bases of the allergic response to different substances of consumption or frequent exposure. Second, to compare the mechanisms explaining the intolerance towards these substances. For example, in the chapter discussing allergy and intolerance to dairy products, we present an immune mechanism mediated by IgE in the case of allergy, while intolerance indicates a metabolic mechanism of enzyme deficiency. However, in both cases there are usually errors or misinterpretation in their knowledge. Lastly, regarding toxicology, the bases of poisoning by these substances will be described when applicable. This is the case of alcohol, for example. The work will be a useful tool for the training of medical personnel in these areas and for students and researchers interested in the investigation of these phenomena.