Various drugs require conjugation with endogenous substrates such as glutathione, glucouronic acid, and sulfuric acid for their inactivation. Consequently, different drugs may compete for the same endogenous substrate, and the faster-reacting drug may effectively deplete the endogenous substrate levels and impair the metabolism of the slower-reacting drug. If the later has a steep dose-response curve or a narrow margin of safety, its pharmacologic and toxic effects may be potentiated.