phase I metabolism The first stage in the conversion of a foreign compound, such as a drug or toxin, into a form that can be eliminated by the body. Common reactions during this phase are oxidation, reduction, and hydrolysis; the resulting metabolites are chemically more reactive than the parent compound, enabling them to undergo the reactions of the second stage (see phase II metabolism).

phase I metabolism The first phase of metabolism of foreign compounds (xenobiotics), involving metabolic activation. Generally regarded as detoxication reactions, but may in fact convert inactive precursors into metabolically active compounds, and involved in activation of precursors to carcinogens.