The serum half-life of a drug is:
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When doctors write prescriptions for medications, they don't just jot down the name of a drug and send their patients off to the pharmacy. Included on the prescription are details about how much of the medication to take at one time (the dose) and at what intervals. These instructions, which are very important for making certain the drug is effective and safe, are based in part on the half-life of the medication being prescribed. Show As a patient, it rarely is necessary to know the half-life of a drug you are taking. But it can help to understand what this term means and how it might affect you during the time you're on the drug. What Is Half-Life?A medication's biological half-life refers simply to how long it takes for half of the dose to be metabolized and eliminated from the bloodstream. Or, put another way, the half-life of a drug is the time it takes for it to be reduced by half. For example, the half-life of ibuprofen (the active ingredient in pain and fever relievers such as Advil and Motrin) is about two hours. This means if you take a dose of 400 milligrams (mg) of ibuprofen at noon, half of the dose (200 mg) will have been eliminated from your bloodstream by 2 p.m. By 4 p.m., another 100 mg will have been eliminated, and so forth. It's important to note that the expected half-life of a drug varies from person to person, depending on factors such as age, weight, genetics, and even specific health issues. For example, the half-life of acetaminophen (the active ingredient in Tylenol), can be significantly affected by a person's liver function, since acetaminophen is primarily processed through the liver. Achieving a Steady StateThe goal of any medication that will need to be taken on an ongoing basis, such as an antidepressant, is to get it to a "steady state"—in other words, to the point at which the amount that goes into the body is equal to the amount that's eliminated. No matter what the half-life of a medication is, it takes about four times that amount of time for the concentration of the drug to reach a steady state in the body. This means that if you begin taking a medication with a half-life of 24 hours, after four days, or on the fifth day, the rate of intake of the drug will approximately equal the rate of elimination. If the half-life is 12 hours, you'll reach a steady state at the beginning of the third day (after 48 hours). Why Half-Life MattersDrugs with a longer half-life take longer to work. But on the positive side, they take less time to leave your bloodstream. Those with a short half-life become effective more quickly, but are harder to come off of. In fact, drugs with very short half-lives can lead to dependency if taken over a long period of time. A drug's half-life is an important factor when it's time to stop taking it. Both the strength and duration of the medication will be considered, as will its half-life. This is important because you risk unpleasant withdrawal symptoms if you quit cold turkey. Withdrawal symptoms are caused by abruptly discontinuing some types of medication. When you stop taking one of these, your doctor will recommend a gradual tapering schedule, taking the drug's half-life into consideration. Those with a longer half-life will have a longer tapering period. Medication side effects occur usually when the blood level of the drug is not in its steady state. That's why it's important to follow dosage and duration recommendations to the letter. Otherwise, the drug will be either toxic (more than intended), or not therapeutic (ineffective for treatment). This is used to measure the removal of things such as metabolites, drugs, and signalling molecules from the body. Typically, the biological half-life refers to the body's natural cleansing through the function of the liver and through the excretion of the measured substance through the kidneys and intestines. This concept is used when the rate of removal is roughly exponential.[clarification needed] 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. This measurement is useful in medicine, pharmacology and pharmacokinetics because it helps determine how much of a drug needs to be taken and how frequently it needs to be taken if a certain average amount is needed constantly. By contrast, the stability of a substance in plasma is described as plasma stability. This is essential to ensure accurate analysis of drugs in plasma and for drug discovery. The relationship between the biological and plasma half-lives of a substance can be complex depending on the substance in question, due to factors including accumulation in tissues (protein binding), active metabolites, and receptor interactions. Examples[edit]The biological half-life of water in a human is about 7 to 14 days. It can be altered by behavior. Drinking large amounts of alcohol will reduce the biological half-life of water in the body. This has been used to decontaminate humans who are internally contaminated with tritiated water. The basis of this decontamination method is to increase the rate at which the water in the body is replaced with new water. Alcohol[edit]The removal of ethanol (drinking alcohol) through oxidation by alcohol dehydrogenase in the liver from the human body is limited. Hence the removal of a large concentration of alcohol from blood may follow . Also the rate-limiting steps for one substance may be in common with other substances. For instance, the blood alcohol concentration can be used to modify the biochemistry of methanol and ethylene glycol. In this way the oxidation of methanol to the toxic formaldehyde and formic acid in the human body can be prevented by giving an appropriate amount of ethanol to a person who has ingested methanol. Note that methanol is very toxic and causes blindness and death. A person who has ingested ethylene glycol can be treated in the same way. Half life is also relative to the subjective metabolic rate of the individual in question. The biological half-life of caesium in humans is between one and four months. This can be shortened by feeding the person prussian blue. The prussian blue in the digestive system acts as a solid ion exchanger which absorbs the caesium while releasing potassium ions. For some substances, it is important to think of the human or animal body as being made up of several parts, each with their own affinity for the substance, and each part with a different biological half-life (physiologically-based pharmacokinetic modelling). Attempts to remove a substance from the whole organism may have the effect of increasing the burden present in one part of the organism. For instance, if a person who is contaminated with lead is given EDTA in a chelation therapy, then while the rate at which lead is lost from the body will be increased, the lead within the body tends to relocate into the brain where it can do the most harm.
Peripheral half-life[edit]Some substances may have different half-lives in different parts of the body. For example, oxytocin has a half-life of typically about three minutes in the blood when given intravenously. 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. 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. In addition, also unlike the case of peripheral administration, intranasal oxytocin has a central duration of at least 2.25 hours and as long as 4 hours. 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. Rate equations[edit]First-order elimination[edit]Timeline of an exponential decay processTime (t)Percent of initial valuePercent completiont½50%50%t½ × 225%75%t½ × 312.5%87.5%t½ × 3.32210.00%90.00%t½ × 46.25%93.75%t½ × 4.3225.00%95.00%t½ × 53.125%96.875%t½ × 61.5625%98.4375%t½ × 70.781%99.219%t½ × 100.098%99.902%Half-times apply to processes where the elimination rate is exponential. If C(t){\displaystyle C(t)}  where k is the reaction rate constant. Such a decay rate arises from a where the rate of elimination is proportional to the amount of the substance: dCdt=−kC.{\displaystyle {\frac {dC}{dt}}=-kC.}The half-life for this process is t12=ln2k.{\displaystyle t_{\frac {1}{2}}={\frac {\ln 2}{k}}.\,}Alternatively, half-life is given by t12=ln2λz{\displaystyle t_{\frac {1}{2}}={\frac {\ln 2}{\lambda _{z}}}\,}where λz is the slope of the terminal phase of the time–concentration curve for the substance on a semilogarithmic scale. Half-life is determined by clearance (CL) and volume of distribution (VD) and the relationship is described by the following equation: t12=ln2⋅VDCL{\displaystyle t_{\frac {1}{2}}={\frac {{\ln 2}\cdot {V_{D}}}{CL}}\,}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. So, for example, digoxin has a half-life (or t½) of 24–36 h; this means that a change in the dose will take the best part of a week to take full effect. For this reason, drugs with a long half-life (e.g., amiodarone, elimination t½ of about 58 days) are usually started with a loading dose to achieve their desired clinical effect more quickly. Biphasic half-life[edit]Many drugs follow a biphasic elimination curve — first a steep slope then a shallow slope: STEEP (initial) part of curve —> initial distribution of the drug in the body.SHALLOW part of curve —> ultimate excretion of drug, which is dependent on the release of the drug from tissue compartments into the blood.The longer half-life is called the terminal half-life and the half-life of the largest component is called the dominant half-life. For a more detailed description see . What is halfDrug half-life is defined as the amount of time required for 50% of a drug to: be eliminated by the body.
Why is the halfUnderstanding the concept of half-life is useful for determining excretion rates as well as steady-state concentrations for any specific drug. 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.
What is plasma halfAbstract. Terminal plasma half-life is the time required to divide the plasma concentration by two after reaching pseudo-equilibrium, and not the time required to eliminate half the administered dose.
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