Eliminating drugs from the body is a vital process that occurs through excretion or metabolism. Understanding the kinetics of drug elimination is crucial for drug development, dosage determination, and optimizing patient outcomes.

Drug clearance depends on the rate of drug elimination and its plasma concentration. Another important parameter is a drug's half-life, which is the time required for its concentration to decrease by half. In most cases, drug clearance follows first-order kinetics, where a constant fraction of the drug is eliminated per unit of time. This means that as the drug concentration increases, its elimination rate also increases. The relationship between drug concentration and time is exponential.

However, certain drugs, like phenytoin, aspirin, omeprazole, and fluoxetine, exhibit zero-order kinetics. When administered at high therapeutic doses, these drugs saturate the enzymes responsible for their metabolism. As a result, only a fixed amount of the drug is eliminated at a constant rate, regardless of its concentration. The plot of drug concentration versus time in zero-order kinetics is linear, indicating that increasing the drug concentration does not affect its elimination rate.

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6.2 : Elimination Kinetics: First-Order and Zero-Order

Pharmacokinetics: Drug Excretion and Clearance

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6.1 : Drug Elimination: Overview

Pharmacokinetics: Drug Excretion and Clearance

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6.3 : Renal Drug Excretion: Overview

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6.4 : Renal Drug Excretion: Glomerular Filtration

Pharmacokinetics: Drug Excretion and Clearance

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6.5 : Renal Drug Excretion: Tubular Reabsorption

Pharmacokinetics: Drug Excretion and Clearance

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6.6 : Renal Drug Excretion: Tubular Secretion

Pharmacokinetics: Drug Excretion and Clearance

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6.7 : Renal Drug Excretion: Effect of Urine pH, Flow Rate, and Drug pKa

Pharmacokinetics: Drug Excretion and Clearance

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6.8 : Hepatic Drug Excretion: Enterohepatic Cycling

Pharmacokinetics: Drug Excretion and Clearance

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6.9 : Hepatic Drug Excretion: Influencing Factors

Pharmacokinetics: Drug Excretion and Clearance

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6.10 : Drug Excretion: Pulmonary and Glandular Routes

Pharmacokinetics: Drug Excretion and Clearance

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6.11 : Drug Excretion: Miscellaneous Routes

Pharmacokinetics: Drug Excretion and Clearance

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6.12 : Drug Clearance: Overview

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6.13 : Clearance Models: Physiological Models

Pharmacokinetics: Drug Excretion and Clearance

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6.14 : Clearance Models: Compartment Models

Pharmacokinetics: Drug Excretion and Clearance

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6.15 : Clearance Models: Noncompartmental Models

Pharmacokinetics: Drug Excretion and Clearance

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