Two drugs having different targets in unrelated pathways that ultimately result in the desired therapeutic result are considered to have additive effects with independent actions. For example, artemisinin and curcumin both exert antimalarial effects. Artemisinin works by being metabolized in the body into active metabolites. The metabolites would then create reactive oxygen species(ROS) that damage the parasites and kill them. The mechanism of action of curcumin remains largely unknown, but the antiparasitic effect is believed to be associated with the potentiation of innate and adaptive immunological responses. The combined effects of artemisinin and curcumin each contribute to the death of parasites via different mechanisms and the effect is shown to be additive by fractional inhibitory concentrations.
Drugs with the same target in different sites that produce additive effects are also considered as independent action. For example, doxorubicin and trabectedin can both produce anticancer effect. Doxorubicin is a DNA intercalator that prefers to bind to AT regions, while trabectedin forms guanine adduct in DNA to disrupt DNA repair system. A recent study has shown that doxorubicin and trabectedin do not hinder each other and could produce an additive anticancer effect.