It is an unintentional effect of using two (or more) drugs, or an interaction between a drug and food, beverage, or supplement. Drug interaction is defined as the change in efficacy or toxicity of one drug by prior or concomitant administration of a second drug or other active substance.

One of the drugs in the combination may alter the behavior of the other drug(s) in the body changing the pharmacokinetics of the drug (altering absorption, distribution, metabolism, or excretion). Instead, drug interactions may result in the competition for a single receptor, or different parts of a signaling pathway.

The factors or conditions that predispose the appearance of interactions include:
• Old age
• Polypharmacy
• Genetic factors
• Hepatic, renal or serious diseases
• Drug dependent factors: narrow therapeutic index, steep dose-response curve, saturable hepatic metabolism.

See reference for more information.

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on May 2, 2019.