When the effects of one drug change due to the presence of another drug/herbal product/medicine, food, drink etc. this is termed as drug interaction. The result can be injurious if the interaction augments the toxicity of the drug. The more drugs a patient takes, the greater the likelihood that an adverse reaction will occur. Usually it is difficult to identify an interaction because of individual variability. Theoretically, many reasons and factors can establish whether an interaction occurs or not. However, practically it is still very difficult to predict the same. The uncomplicated way out to this practical problem is to choose a non-interacting alternative with precaution, however, it is not always possible. The level of interaction can be minimized in certain cases intelligibly by adjusting the dosages. In this way, the dose-related interactions can be reduced. It is reasonably impracticable to retain information and occurrence about all the known clinically important interactions that is why this book about analgesics has been produced.