Pharmacokinetics describes how a medication, supplement, or chemical compound in general moves through and is transformed within the body, from the point of administration until elimination. Simply put, pharmacokinetics looks at what the body does to a medication, as opposed to pharmacodynamics, which focuses on what a medication does to the body. The pharmacokinetics of a given medication or supplement depends on the characteristics of the compound (e.g., its solubility, size, pH), the dose and route of administration, and the functioning of the body.^[1][2]

There are four main parameters defining pharmacokinetics: absorption, distribution, metabolism, and excretion. Collectively, these parameters are often called ADME.^[1] If a compound interferes with any of these processes, a pharmacokinetic drug interaction can occur.^[3]

Absorption

Absorption describes the process of a compound moving from the site of administration (e.g., gastrointestinal tract, nasal cavity, skin) to the systemic blood circulation, and it is often characterized by bioavailability.^[2]

Many factors can influence absorption. For example, the absorption of a medication taken orally can be influenced by the pH of the gastrointestinal tract, the presence of food or other medications, gastrointestinal motility, disease affecting the intestinal tract (e.g., celiac disease, inflammatory bowel disease), digestive secretions and enzymes, and the activity of transporters (cell membrane components that help to transport molecules in or out of the cell, e.g., p-glycoprotein, organic anion transporting polypeptides).^[2]

Distribution

After administration and absorption, a substance is distributed among the body’s tissues, including the bloodstream, organs, fat, and other body tissues.^[1]

Distribution can be impacted by body composition, rate of blood perfusion through tissues, and the degree to which the medication is bound to proteins in circulation (e.g., albumin, sex hormone binding globulin).^[2]

Metabolism

Metabolism involves the enzymatic transformation of a compound, typically into a form that is inactive and easier for the body to excrete. However, in some cases, the compound may be metabolized into a more biologically active form. This process occurs in various locations throughout the body, with the liver being the primary site.^[1]

Anything that influences the activity of drug-metabolizing enzymes, most notably those of the cytochrome P450 family, can influence drug metabolism. This includes genetics, age, lifestyle factors (e.g., smoking, diet), disease states (e.g., liver disease, inflammation), and co-administration of other medications/supplements.^[4]

Excretion

Excretion describes the elimination of a compound from the body. The major routes of excretion involve the kidneys and liver, via urine and feces, respectively, and the health of these organs can severely impact excretion rates. Minor routes of excretion include sweat, the lungs, and breastmilk.^[1]