When you take medicine, it embarks on a complex journey through your body. Understanding this process, known as pharmacokinetics, helps explain how medicines work and why their impact varies. It encompasses how the body takes in the medicine, moves it around, chemically changes it, and finally gets rid of it.
How Medicine Enters and Travels
Medicine begins its journey by moving from its administration site into the bloodstream, a process called absorption. The route of administration influences how quickly and efficiently this happens. For instance, oral medications, like a swallowed tablet, must first dissolve in the stomach or intestine before passing through the intestinal wall into the blood vessels. In contrast, intravenous injections deliver the medicine directly into the bloodstream, bypassing absorption entirely and resulting in immediate and complete availability. Other common routes include intramuscular injections, transdermal patches, or inhalation, each with distinct absorption characteristics.
Once absorbed, the medicine is then distributed throughout the body via the bloodstream. Blood circulates to all tissues and organs, carrying the drug to its target site where it can exert its therapeutic effect. During this distribution, some medicines may temporarily bind to proteins in the blood. This binding can affect how much of the active drug is immediately available to reach its target, as only the unbound portion can interact with tissues.
Transforming the Medicine
Following distribution, the body begins to chemically alter the medicine through a process called metabolism. The liver is the primary organ responsible for this transformation, although other organs like the intestines, kidneys, and lungs also contribute. Metabolism often changes the drug into new forms, called metabolites, which can be more easily eliminated from the body.
Specific enzymes, particularly the Cytochrome P450 (CYP450) family of enzymes, play a central role in these chemical reactions. A few key CYP450 enzymes, like CYP3A4 and CYP2D6, metabolize most drugs. Metabolism can lead to several outcomes: it might inactivate the drug, reduce its activity, or, in some cases, activate a “prodrug” that was initially inactive. Metabolism often makes drugs more water-soluble, aiding their removal.
Removing Medicine from the Body
Excretion is the process where the body eliminates drugs and their metabolites. The kidneys are the main organs for removing water-soluble drugs and their byproducts from the bloodstream, ultimately expelling them in urine. The liver also plays a significant role, secreting some drugs and metabolites into bile, which then enters the digestive tract and is removed with feces.
Other pathways for excretion exist, accounting for smaller amounts. These include exhalation through the lungs for volatile compounds, and through sweat, saliva, or breast milk. Proper excretion prevents drug buildup, which could lead to side effects or toxicity.
Individual Differences in Processing
The way a body processes medicine can vary from person to person. Genetic variations can influence the activity of drug-metabolizing enzymes, affecting how quickly drugs are broken down. Some individuals might be “fast metabolizers,” processing drugs more rapidly, while others might be “slow metabolizers,” resulting in higher drug levels.
Age also affects drug processing; infants and young children may have immature organ systems that process drugs differently, while older adults may experience reduced liver and kidney function, which can slow down metabolism and excretion. Organ health, especially of the liver and kidneys, impacts the body’s ability to metabolize and eliminate medicines. Conditions like liver disease or kidney disease can impair these processes, potentially requiring adjustments to drug dosages.
Interactions with other medications can influence how drugs are processed. One drug might enhance or inhibit the activity of enzymes responsible for another drug’s metabolism, altering its effectiveness or increasing the risk of side effects. Lifestyle factors, including diet and smoking, can also affect drug metabolism; for instance, certain foods like grapefruit juice can inhibit specific drug-metabolizing enzymes, and smoking can induce others.