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How can drug metabolism and excretion lead to harmful drug interactions?

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Harmful drug interactions often arise when one medication alters the metabolism or excretion of another, potentially leading to toxic concentrations or reduced therapeutic effectiveness.1

Metabolic Interactions

Drug metabolism primarily occurs in the liver via enzymes. Interactions occur when:
* Enzyme Inhibition: One drug inhibits the enzymes responsible for breaking down a second drug. This causes the second drug to remain in the body longer, increasing its concentration and risk of toxicity.1
* Enzyme Induction: Some drugs increase the production of metabolic enzymes, which accelerates the breakdown of other medications. This can lower the body concentration of the second drug, rendering it therapeutically ineffective.1
* Competition: Multiple drugs may compete for the same metabolic enzyme, which is a common source of adverse interactions.2

Excretion Interactions

Excretion primarily occurs through the kidneys. If one drug alters kidney function, it can inadvertently increase or decrease the rate at which another drug is eliminated from the body, leading to unpredictable levels of the medication in the system.1