text: The concept of additive effect is analogous to the concept of simple addition in mathematics. However, the additive effect is not simply the arithmetic summation of two (or more) drugs in most cases. For an additive inhibition effect, drug A and drug B could each inhibit 20% individually, but the additive effect is not 40%. The effect cannot be simply arithmetic because if drug A and drug B each inhibits 60% cannot theoretically exert an inhibitory effect of 120%. With 60% inhibitory effect each, the remaining function would be at (1-60%)×(1-60%)=16%, meaning the additive inhibitory effect would be 84%. Since the application of additive effect is commonly seen in clinical practice, avoiding the common misconceptions of additive effect is crucial to understanding the clinical significance of additive effect.