Interactions of Bioactive Plant Metabolites: Synergism, Antagonism, and Additivity

Drugs are commonly used in mixtures, also called cocktails, to treat disease, particularly cancer and viral infections. Any two or more drugs, or for that matter, two or more bioactive plant compounds, will either interact in some way or fail to interact. If an interaction produces an effect greater than that expected for each individual drug, the interaction is termed synergistic. If the effect is less than expected, it is termed antagonistic. If the effect is equal to the expected effect (i.e., there is no interaction), the interaction is termed additive (see Greco et al., 1995; Spelman, 2007, in Cseke et al., 2006). In most therapeutic situations, the hope is that mixtures will produce a synergistic effect, but additivity can also be useful and should not be neglected.