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How can drug metabolism make a drug less active or more toxic?

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Drug metabolism, or biotransformation, is the process by which the body modifies chemicals—primarily in the liver—to facilitate their elimination.21

Reducing Activity

In most instances, metabolic reactions catalyzed by enzymes in the endoplasmic reticulum transform a parent drug into metabolites that are less active and more water-soluble, allowing them to be excreted more rapidly in urine or bile. This process effectively terminates the drug's biological activity.21

Increasing Toxicity

Biotransformation is not always beneficial, as some metabolic pathways can produce substances more harmful than the original drug. This occurs because:21
* Reactive Intermediates: Enzymes such as cytochrome P-450 monooxygenases can create highly reactive products, such as epoxides, which may damage cellular components like DNA.1
* Toxic Metabolites: Certain drugs are converted into specific metabolites that cause organ damage. For example, research has identified that specific metabolites of acetaminophen are responsible for the liver toxicity observed in overdose cases.3

Ultimately, the balance between detoxification and the formation of toxic intermediates determines the safety profile of many chemical agents.21