What is the Half-Life of a Drug?

By Yolanda Smith, B.Pharm.Reviewed by Dr. Liji Thomas, MD

The elimination half-life of a drug is a pharmacokinetic parameter that is defined as the time it takes for the concentration of the drug in the plasma or the total amount in the body to be reduced by 50%. In other words, after one half-life, the concentration of the drug in the body will be half of the starting dose.

With each additional half-life, proportionately less of the drug is eliminated. However, the time required for the drug to reach half of the original concentration remains constant.

In general, the effect of the drug is considered to have a negligible therapeutic effect after 4 half-lives, that is, when only 6.25% of the original dose remains in the body.

Illustrative example

In the event that a 100 milligram (mg) dose of an intravenous drug with a half-life of 15 minutes is administered, the following would be true:

• 15 minutes after the drug administration, 50 mg of the drug remains in the body.
• 30 minutes after the drug administration, 25 mg of the drug remains in the body.
• 45 minutes after the drug administration, 12.5 mg of the drug remains in the body.
• 1 hour after the drug administration, 6.25 mg of the drug remains in the body.
• 2 hours after the drug administration, 0.39 mg of the drug remains in the body.

Related terms

There are several other terms that are closely related to the drug half-life, including:

• The elimination rate constant (λ): This describes the rate of drug elimination from the body as a fraction. This value is constant in first-order kinetics and independent of drug concentration.
• Apparent half-life: In some circumstances, such as controlled-release formulations, the decline in the concentration of the drug is not solely dependent on elimination, but also on the rate of absorption and distribution, which influences the observed half-life.

Clearance and volume of distribution

There are two factors that affect the elimination half-life of a drug, which include its clearance and volume of distribution. The clearance of the drug (CL) refers to the rate at which the body eliminates the drug from the body. Alternatively, the volume of distribution (Vd) refers to the distribution of the drug around the body.

The relationship between CL and Vd is as follows:

t(½) = ln(2)/ λ = ln(2)*(Vd/CL) = 0.693*(Vd/CL)

The elimination half-life is considered to be constant and independent of the concentration of the drug in the body.

Clinical uses

The elimination half-life is a useful pharmacokinetic parameter, as it provides an accurate indication of the length of time that the effect of the drug persists in an individual.

Moreover, the elimination half-life can also show if accumulation of the drug is likely to occur with a multiple dosing regimen. This is helpful when it comes to deciding the appropriate dose and frequency of a prescribed drug.

Along with other pharmacokinetic data and values about the individual patient, the half-life can help health practitioners to estimate the rate at which a drug will be eliminated from the body, as well as how much will remain after a given time period. From this information, appropriate decisions can be made on how to promote patient health outcomes.

References

• http://www.ncbi.nlm.nih.gov/books/NBK12815/
• http://sepia.unil.ch/pharmacology/?id=60
• http://pharmacologycorner.com/definition-of-half-life-of-drugs/
• http://www.annualreviews.org/doi/abs/10.1146/annurev.me.36.020185.002225?journalCode=med
• http://www.usada.org/drug-half-life/

Further Reading

• All Pharmacokinetics Content
• Pharmacokinetics