Phenomenon in which two or more objects have an effect upon one another.

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 medications fall generally into one of two main categories:

In terms of efficacy, there can be three types of interactions between medications: additive, synergistic, and antagonistic.

The word epistasis is also used for genetic interaction in some contexts.