Characteristics of phase I drug metabolizing reactions include conversion of the parent drug into a less polar, more lipid soluble metabolite.

Phase I drug metabolizing reactions often introduce functional groups such as -OH, -SH, or -NH2 into the parent drug.

Metabolites generated by phase I drug biotransformation may exhibit:

Concerning phase I drug biotransformation:

Phase II metabolism is characterized by "conjugation or synthetic" reactions.

Isoniazid (INH,isonicotinylhydrazide), at the hydride moiety, is directly conjugated forming an N-acetyl form, initially skipping entirely phase I metabolism.

The N-acetyl conjugate of isoniazid undergoes a phase I hydrolysis reaction yielding isonicotinic acid. In this example, a phase II reaction precedes the phase I reaction.

The principal organ for drug metabolism:

Midazolam (Versed) is about 50% metabolized in the intestine.

Drugs that are absorbed from the small intestine and transported prior to any metabolic change via the portal system to the liver and there undergo substantial metabolism are said to exhibit a first-pass effect.