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. In molecular biology, the knowledge on gene/protein interaction among themselves and with their metabolites is referred to as molecular pathways.

Interactions between medications (drug interactions) fall generally into one of two main categories:

  1. pharmacodynamic : Involving the actions of the two interacting drugs.

  2. pharmacokinetic : Involving the absorption, distribution, metabolism, and excretion of one or both of the interacting drugs upon the other.

In terms of efficacy, there can be three types of interactions between medications: additive, synergistic, and antagonistic. Additive interaction means the effect of two chemicals is equal to the sum of the effect of the two chemicals taken separately. This is usually due to the two chemicals acting on the body in the same way. Examples would be Aspirin and Motrin, Alcohol and Depressant, Tranquilizer and Painkiller. Synergistic interaction means that the effect of two chemicals taken together is greater than the sum of their separate effect at the same doses. An example is Pesticide and Fertilizer, the biological effect is devastating. Antagonistic interaction means that the effect of two chemicals is actually less than the sum of the effect of the two drugs taken independently of each other. This is because the second chemical increases the excretion of the first, or even directly blocks its toxic actions. Antagonism forms the basis for antidotes of poisonings.