Thus, it follows thatВ after 4В to 5 half-lives, the plasma concentrations of a given drug will be below a clinically relevant concentration and thus will be considered eliminated. Even further, 94 to 97% of a drug will have been eliminated after 4 to 5 half-lives. For example, 90% of a given drug will have undergone elimination after approximately 3.3 half-lives. Assuming no administration of additional drug after an initial dose, ignoring any drug-drug interactions, and assuming a physiologically healthy individual, certainВ quantitative constants apply to all drugs exhibiting first-order pharmacokinetics. It is also worth discussingВ the relationship between the percentage of drug eliminated and the number of half-lives. Д®quation 3: t-half= 0.693*Vd/CL, where Vd is the volume of distribution and CL is clearance An alternative half-life equation exists that relates half-life to other pharmacokinetic parameters known as the volume of distribution and clearance (Equation 3). From this equation, one can quickly determine the half-life of a drug, given its predetermined rate constant k. EliminationВ curvesВ are usefulВ for determining if a drug indeed follows first-order kinetics, in which case the curve shouldВ followВ a logarithmic decayВ according to the integrated rate law of first-orderВ reactions (Equation 1).В After solving the differential equation, one can obtain the half-life equation that is commonly tested on and used in clinical practice (Equation 2). Total drug exposure over time is represented in these graphs as the integral area under the curve (AUC). Half-life elimination isВ graphically representedВ with elimination curves that track the amount of a drug in the body over time, typically with timeВ on the independent axis and drug plasma concentrationВ on the dependent axis, as shown in Figure 1. This article will focus on first-order half-life elimination as it is the most frequently encountered in clinical practice. In contrast, a few drugs follow zero-order elimination in which the drug amount decreases by a constant amount over time regardless of initial concentration (i.e., ethanol). Most clinically relevant drugs tend toВ follow first-order pharmacokinetics that is, their drug-elimination rates are proportional toВ plasma concentrations. The characteristic decreases of drugs over time have long been studied inВ a field known as pharmacokinetics and are depictable by basicВ differentialВ equations. Different drugs have different half-lives however, they all follow this rule: after one half-life has passed, 50% of the initial drug amount is removed from the body. Understanding the concept of half-lifeВ is useful for determining excretion rates as well as steady-state concentrations for any specific drug. Half-life in the context of medical science typically refers to the elimination half-life.В The definition of elimination half-life is the length of time required for the concentration of a particular substance (typically a drug) to decreaseВ toВ half of itsВ starting dose in the body.

What is a half life