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Half-life (symbol t ½) is the time required for a quantity (of substance) to reduce to half of its initial value.The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable atoms survive.
Absorption half-life 1 h, elimination half-life 12 h. Biological half-life (elimination half-life, pharmacological half-life) is the time taken for concentration of a biological substance (such as a medication) to decrease from its maximum concentration (C max) to half of C max in the blood plasma.
But is also equivalent to divided by elimination rate half-life /, = /. Thus, = /. This means, for example, that an increase in total clearance results in a decrease in elimination rate half-life, provided distribution volume is constant.
A more intuitive characteristic of exponential decay for many people is the time required for the decaying quantity to fall to one half of its initial value. (If N(t) is discrete, then this is the median life-time rather than the mean life-time.) This time is called the half-life, and often denoted by the symbol t 1/2. The half-life can be ...
It was proposed to be used instead of AUC in animal-to-human dose translation, as computer simulation shows that it could cope better with half-life and dosing schedule variations than AUC. This is an example of a PK/PD model , which combines pharmacokinetics and pharmacodynamics .
An effective half-life of the drug will involve a decay constant that represents the sum of the biological and physical decay constants, as in the formula: = + With the decay constant it is possible to calculate the effective half-life using the formula:
The half-life, t 1/2, is the time taken for the activity of a given amount of a radioactive substance to decay to half of its initial value. The decay constant , λ " lambda ", the reciprocal of the mean lifetime (in s −1 ), sometimes referred to as simply decay rate .
The absorption rate constant K a is a value used in pharmacokinetics to describe the rate at which a drug enters into the system. It is expressed in units of time −1. [1] The K a is related to the absorption half-life (t 1/2a) per the following equation: K a = ln(2) / t 1/2a.