Drug interaction is defined as an interaction between a drug and another substance that prevents the drug from performing expected effects, the outcome of which can be synergistic, antagonistic, or idiosyncratic. So the effects of drug interactions can be beneficial or harmful. This definition includes- drug-drug interactions, drug-food interactions and interactions between drugs and other substances. Interactions among drugs may affect the desired result of taking medications by the patients. So, during prescribing drugs for the patients- interactions among these drugs should be kept in the mind of the physician. To have some examples of Drug-Drug interactions click here.