It details the potential interactions associated with usage of a medicinal product. It occurs when two or more medicinal products react with each other. This interaction may cause the person taking that medicinal product to experience an unexpected side effect. For example, mixing a drug one takes to help one sleep (a sedative) and a drug one takes for allergies (an antihistamine) can slow his/her reactions and make driving a car or operating machinery dangerous.
Sometimes, the interaction may result from medicinal products reacting with foods or beverages. For example, mixing alcohol with some medicine may cause a person to feel tired or slow his/her reactions. In addition to the above interactions, another kind of interaction may occur when an existing medical condition makes certain drugs potentially harmful. For example, if a person has high blood pressure that person could experience an unwanted reaction if he/she takes a nasal decongestant.