Figure 6.

The synergistic effect of the two drugs might be influenced by the metabolic course. When a pair of drug compounds at the desired ratio (demonstrated for synergistic effect in vitro) was orally administered into body, the metabolic processes of drug absorption, distribution, metabolism, and excretion will change the ratio of the drugs. As a consequence, the in vivo synergistic effects of the drug pair may disappear if the drug pair can only possess a synergistic ratio within a narrow range.