Which phase of drug metabolism occurs first after drug ingestion?

Phase I of metabolism is the initial stage of drug metabolism following drug ingestion. This phase primarily involves the chemical modification of the drug through processes such as oxidation, reduction, and hydrolysis, typically mediated by enzymes from the cytochrome P450 family.

The main goal of Phase I metabolism is to convert lipophilic compounds into more hydrophilic metabolites that can be more easily excreted from the body. These Phase I reactions often generate active metabolites, which can sometimes contribute to the pharmacological effects of the drug or may be further processed in the following Phase II metabolism.

Understanding Phase I metabolism is crucial for predicting how different drugs will behave in the body, including their activation, toxicity, and interaction with other medications. This foundational phase lays the groundwork for subsequent metabolism and elimination, making it a critical point in pharmacokinetics.