American statistics reveal that adverse drug reactions occur in one out of fifteen people taking drugs. It is estimated that approximately 2,2 million people suffer drug related adverse reactions annually. Drug interactions account for a third of drug events and half of the total cost imputed and related to drug injury. Considering all types of drug interactions, it is estimated that as low as 3% to 5% of patients taking a small number of drugs to as high as 20% of those taking 10 to 20 medications may be affected by a drug interaction. Though constellation of drug interactions may happened and supervened, the actual number of serious events are rather modest and manageable. The clinician must anticipate and must use his judgment to evaluate concomitant prescribed drugs, over-the-counter medications, environmental factors and comorbidities.