An overview of how medications work in the body
Pharmacokinetics is the study of how drugs travel through the body and influence their effects. It is often summarized by four key processes: absorption, distribution, metabolism, and excretion. Understanding these steps helps explain why some medications act quickly, why doses differ between people, and why drug interactions can occur.
Absorption is the process by which a drug moves from its site of administration, such as the stomach, skin, or bloodstream, into circulation. The rate of absorption determines how quickly a drug begins to work, while the amount absorbed influences the intensity of its effect. Factors that affect absorption include the drug’s dissolution rate, surface area for absorption, blood flow, lipid solubility, pH balance, and the presence of transport proteins that either aid or block drug entry.
Once in the bloodstream, drugs undergo distribution, which is the process of spreading throughout the body. Drugs move into different compartments, such as tissues, fat, or organs, where they may be stored, metabolized, excreted, or trigger their intended effects. Distribution depends largely on blood flow, the ability of drugs to pass through capillary walls, and whether they can enter target cells.
Metabolism refers to the chemical modification of drugs, usually by enzymes. The liver is the primary site of metabolism, though the stomach, intestines, and kidneys also contribute. Metabolism plays a protective role by breaking down potentially harmful substances while also creating essential molecules the body needs. Importantly, metabolism can either activate or inactivate drugs.
Finally, excretion removes drugs and their metabolites from the body. The kidneys are the main route of drug elimination, filtering substances into urine and determining how long a drug remains active. Other pathways include excretion through bile, sweat, saliva, breast milk, lungs, and even hair.
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This article is not intended to be a substitute for professional medical advice or diagnosis. Always seek the advice of your physician or another qualified health provider with any questions you may have regarding a medical condition.
