Biological half-life
Typically, the biological half-life refers to the body's natural detoxification (cleansing) through liver metabolism and through the excretion of the measured substance through the kidneys and intestines.
[6] In a medical context, half-life explicitly describes the time it takes for the blood plasma concentration of a substance to halve (plasma half-life) its steady-state when circulating in the full blood of an organism.
Drinking large amounts of alcohol will reduce the biological half-life of water in the body.
For instance, the blood alcohol concentration can be used to modify the biochemistry of methanol and ethylene glycol.
Peripherally administered (e.g. intravenous) peptides like oxytocin cross the blood-brain-barrier very poorly, although very small amounts (< 1%) do appear to enter the central nervous system in humans when given via this route.
[31] In contrast to peripheral administration, when administered intranasally via a nasal spray, oxytocin reliably crosses the blood–brain barrier and exhibits psychoactive effects in humans.
[34][35] In likely relation to this fact, endogenous oxytocin concentrations in the brain have been found to be as much as 1000-fold higher than peripheral levels.
Such a decay rate arises from a first-order reaction where the rate of elimination is proportional to the amount of the substance:[39] The half-life for this process is[39] Alternatively, half-life is given by where λz is the slope of the terminal phase of the time–concentration curve for the substance on a semilogarithmic scale.
[40][41] Half-life is determined by clearance (CL) and volume of distribution (VD) and the relationship is described by the following equation: In clinical practice, this means that it takes 4 to 5 times the half-life for a drug's serum concentration to reach steady state after regular dosing is started, stopped, or the dose changed.
For this reason, drugs with a long half-life (e.g., amiodarone, elimination t1/2 of about 58 days) are usually started with a loading dose to achieve their desired clinical effect more quickly.
