Drug interactions (DIs) represent an important and widely under recognized source of medication errors. An interaction is said to occur when the effects of one drug are changed by the presence of another drug(s), food, drink or an environmental chemical. When a therapeutic combination could lead to an unexpected change in the condition of the patient, this would be described as an interaction of potential clinical significance. DIs can arise in numerous ways; such as pharmacodynamic interaction, in which receptor effects of different agents interacts to produce synergy or antagonism of drug effects. In pharmacokinetic interaction, the blood levels of given agents may be raised or lowered based on the type of interaction. Special attention and thorough monitoring is needed for the patients who are predisposed to develop DIs and those on drugs with narrow therapeutic index. DIs can be a very important contributory factor for the occurrence of adverse drug reactions and adverse drug events. DIs monitoring programs should be initiated and strengthened in order to minimize their occurrence. Herbal drug interactions and DIs comprising over the counter medicines should also be considered seriously.