Generally, an interaction is a kind of action which occurs as two or more objects have an effect upon one another.
The word is sometimes used synonymously with "transaction"; in some cases, a difference between the two should be distinguished.
Examples include: the interaction between a driver and the position of his or her car on the road: by steering the driver influences this position, by looking this information returns to the driver; the feedback during operation of other machines such as a computer or a tool; two people talking to each other (sometimes the interaction is not satisfactory, because one or both do not listen well).
In communications, interactive communication occurs when sources take turns transmitting messages between one another. This should be distinguished from transactive communication, in which sources transmit messages simultaneously.
In medicine, most medications can be safely used with other medicines but particular combinations of medicines need to be monitored for interactions, often by the pharmacist.
Interactions between drugs fall generally into one of two main categories; pharmacodynamic (involving the actions of the two interacting drugs), and pharmacokinetic (involving the absorption, distribution, metabolism, and excretion of one or both of the interacting drugs upon the other).
Sometimes two or medications are used together to create an extra effect - e.g. two different pain killers to provide more complete pain control. These interactions are usually intentional but need to be monitored by the doctor because patients can end up with more effect than is actually required.
Sometimes two or more medications work against each other. These interactions are usually well-known and avoided unless both medicines are essential. Careful monitoring is used to prevent problems from the results of the interaction.
Other interactions may cause one medicine to have less or more effect than expected and these are usually managed by a dosage adjustment.
