A drug-drug interaction (DDI) is a pharmacokinetic or pharmacological influence of one medication on another that differs from the known or anticipated effects of each agent alone. A DDI may result in a change in either drug efficacy or drug toxicity for one or both of the interacting medications. Pharmacokinetic DDIs result in altered absorption, distribution, metabolism, or excretion of a medication. A pharmacodynamic DDI occurs when one medication modifies the pharmacological effect of another in an additive, a synergistic, or an antagonistic fashion.  

The overall clinical impact of a DDI can range from mild to life-threatening, therefore, not all DDIs require a modification in therapy. The variability in the clinical significance of a DDI depends on both medication-specific and patient-specific factors.