Even if clinical and laboratory parameters can be considered as predictors for ADRs, the possibility of a drug-related variation of blood tests is seldom taken into consideration. Aim of this easy-to-read book is to help physicians in the routine interpretation of laboratory results, drawing their attention to the possibility that abnormal laboratory results may be drug-related. The book describes the most common variations (increase/decrease) of blood parameters that can be caused by drugs intake. Functions of each blood parameter are schematically reported, together with its standard blood concentration and a list of the most common disease for whose diagnosis that test is performed. Active principles are then listed, that can cause an increase or a decrease of that parameter.