Multiple drug administration increase chances of drug interaction, altering the responses of drugs. Generally multiple drug administration should be avoided for safe and cost-effective patient care. However in major parts of the world there is augmented load of cardiovascular disease, epilepsy and chronic non-communicable disorders such as diabetes mellitus. Hence it is essential to recognize the probable toxicities that might take place as a result of the drug-drug interaction upon simultaneous use of anti-hypertensive, anti-arrhythmic, anti-epileptic, and anti-diabetic drugs. This monograph provides useful information of biochemical, hematological and histopathological changes which might occur following administration of multiple drugs, hence it may serve as an effective tool to investigate the toxicities of multiple drug administration. This monograph is equally beneficial for both graduate and postgraduate students of pharmacology and toxicology to gain a better understanding about the probable mechanisms involved in cumulative toxicities.