Fig 3. The change in the plasma concentration of active ingredient depends on drug type.

Assuming the body is one compartment and the rate of drug elimination in the plasma is constant, when a drug is administered to the human body, it undergoes absorption, metabolism, distribution, excretion, and resorption. In this stage, the plasma concentration of an active ingredient is decreased step by step. However, in the case of a prodrug, after absorption and metabolism, the plasma concentration of an active ingredient appears, and the next steps are the same as those of the active drug.