Drug half-life is defined as the amount of time required for 50% of a drug to:
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The half-life (t1/2) of a drug is the time required for the amount of drug in the body or blood to fall by 50%. It is only applicable to drugs that exhibit first-order kinetics, in which a constant fraction of drug is eliminated per unit time as shown below. In zero-order kinetics, a constant amount of drug is eliminated per unit time. Show
T1/2 can be determined if the clearance (Cl) and volume of distribution (Vd) is known. Cl is the ratio of the rate of elimination of a drug to the concentration in the plasma (rate of elimination/plasma drug concentration). The Vd is the ratio of the amount of drug in the body to the drug concentration in the plasma (amount of drug in body/plasma drug concentration). The half-life of a drug can be determined using the following equation: t1/2 = (0.7 times Vd) / Cl Therefore, t1/2 = (0.7 times 40L) / 2.0 L/hour, and t1/2 = 14 hours. Note: 0.7 is a commonly used log approximation, but not the actual value. Another commonly used approximation is 0.693 for -ln(0.5) = 0.69315.  The half-life determines the rate at which a drug concentration rises during a constant infusion and also the rate at which the concentration falls after drug administration is stopped. It is commonly accepted that it takes four to five half-lives to reach steady state, as shown in the figure to the right. Tips to remember Tips to remember
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series. The AMA selected Kaplan as a preferred provider to support you in reaching your goal of passing the USMLE® or COMLEX-USA®. AMA members can save 30 percent on access to additional study resources, such as Kaplan’s Qbank and High-yield courses. Learn more. The half-life of a drug is the time taken for the plasma concentration of a drug to reduce to half its original value. Half-life is used to estimate how long it takes for a drug to be removed from your body. For example: The half-life of Ambien is about 2 hours. So if you take Ambien after 2 hours the plasma concentration will be reduced to half, after 2 more hours the remaining blood levels will be reduced by another half - so a quarter will be left. Two hours later another half will be removed leaving one eighth and so on. Generally it is considered that it takes 5.5 half-lifes for a drug to be removed from the body, in that it is considered to no longer have a clinical effect. So for Ambien it would take approximately 11 hours (2 hours X 5.5) to be eliminated from your body. Elimination of a drug varies from person to person due to factors like age, weight, other medications taken, or other medical conditions present, also kidney function, liver function etc. Therefore half-life is used as a guide or an estimate of how long it may take for the drug to be removed from the body. Half-life calculator | What is half-life | Patient-specific variables | Drug-specific variables | Short vs long half-life | Common substances and their half-lives Drug Half Life / Clearance CalculatorWork out how long it takes for a drug to leave your body. This is an estimate on the time it will take for a drug to be removed from the body. The actual half-life of the same drug may vary significantly from person to person, because it depends on a number of different patient-specific and drug-specific variables. This tool should NOT be considered as a substitute for any professional medical service, NOR as a substitute for clinical judgement. It should not be relied upon to predict the time period required to ensure a negative drug test result, because laboratory tests mostly test for drug metabolites. What is the half-life of a Drug?The half-life of a drug is an estimate of the time it takes for the concentration or amount in the body of that drug to be reduced by exactly one-half (50%). The symbol for half-life is t½. For example, if 100mg of a drug with a half-life of 60 minutes is taken, the following is estimated:
In theory, we can see that after 300 minutes, almost 97% of this drug is expected to have been eliminated. Most drugs are considered to have a negligible effect after four-to-five half-lives. However, this does not mean that won’t be detectable, for example, during a drug test. Just that they will have no effect. In reality, the actual half-life of a drug varies from person to person, because it depends on a number of different patient- and drug-specific factors. These affect how well a particular drug is distributed around a person’s body (called the volume of distribution), or how fast a person excretes that drug (called the drug clearance). For example, the IV drug gentamicin, which is cleared through the kidneys, has a half-life of 2-3 hours in a young person with no kidney disease, but its half-life is over 24 hours in somebody with severe kidney disease. Generally, it is difficult to precisely say how long a drug or substance will take to be excreted from someone’s body. This is an important fact for athletes or people in occupations that require them to be substance-free to remember. Half-lives in the anti-doping world are of limited value because they do not reflect the presence of metabolites (break-down products from the parent drug), which are often what is measured in anti-doping tests. In addition, serum half-life does not necessarily reflect urine concentrations, which is the main way they take samples for drug testing. Patient-specific variables that may affect half-life
Drug-specific variables that may affect half-life
Short versus long half-livesDrugs or substances that have a shorter half-life tend to act very quickly, but their effects wear off rapidly, meaning that they usually need to be taken several times a day to have the same effect. Drugs with a longer half-life may take longer to start working, but their effects persist for longer, and they may only need to be dosed once a day, once a week, once a month, or even less frequently. When considering drugs with a high addiction or dependence potential, those with a short half-life are typically harder to withdraw from than those with a long half-life. For this reason, drug treatment programs will often switch a person from a short-acting drug to a long-acting equivalent from the same class, in order to improve the withdrawal process. List of common medicines or substances and their half-lives* Note that half-life varies depending on the source used. Half-life in this table refers to the elimination half-life. See alsoFurther informationAlways consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances. What is the meaning of halfThe half-life of a drug is the time it takes for the amount of a drug's active substance in your body to reduce by half. This depends on how the body processes and gets rid of the drug. It can vary from a few hours to a few days, or sometimes weeks.
How is halfThe half-life (t1/2) is the time it takes for the plasma concentration of a drug or the amount of drug in the body to be reduced by 50%. The half-life of a drug can be determined using the following equation: t1/2 = (0.7 x Vd) / Cl, where Vd is volume of distribution and Cl is clearance.
What is the halfDrug half-life is defined as the amount of time required for 50% of a drug to: be eliminated by the body.
What is 5.5 halfGenerally it is considered that it takes 5.5 half-lifes for a drug to be removed from the body, in that it is considered to no longer have a clinical effect. So for Ambien it would take approximately 11 hours (2 hours X 5.5) to be eliminated from your body.
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