Drugs
The half-life of the drug is useful in determining the dosing frequency.

Drug Half-life (t1/2) is defined as the amount of time required for the drug concentration to be reduced to exactly half its initial concentration or amount in the blood. The half-life is the time required for half of the drug to be metabolized and eliminated from the body. The half-life of the drug is useful in determining the dosing frequency. The factors that affect the half-life include:

  • Age
  • Blood circulation
  • Diet (grapefruit juice and several drugs, green vegetables, and warfarin)
  • Excessive fluid (such as in people with heart failure or generalized swelling) or dehydration (low fluid levels)
  • Gender
  • History of drug use
  • Kidney function (for drugs that are cleared through the kidneys)
  • Liver function (for drugs that are metabolized through the liver)
  • Being overweight
  • Pre-existing conditions (such as heart failure, gastrointestinal disorders, and pregnancy)
  • Presence of drugs that compete for binding sites or interact in other ways
  • Race/ethnicity (this can influence the metabolism of a drug)
  • Smoking
  • Other variables, such as if the person is on hemodialysis

The study of half-life reflects a measure of pharmacokinetics. Pharmacokinetics refers to what the body does to a drug or the movement of drugs through the body. It is the study of absorption, distribution, metabolism, and elimination of the drug. Half-lives are commonly used in pharmacokinetics to describe drug absorption and elimination. As a consumer, it is essential to be aware of the half-lives of the drug.

What is the half-life of some drugs?

The table summarizes the half-lives of some common drugs.

Table. Some common drugs and their half-lives
Generic NameHalf-life (T1/2)
Alprazolam6-12 hours
Amiodarone15-142 days
Amphetamine10-12 hours
Atenolol6-7 hours
Clonazepam18-50 hours
Cocaine50 minutes
Diazepam20-100 hours
Donepezil70 hours
Dutasteride5 weeks
Erenumab28 days
Fluoxetine2-4 days
Heroin2-6 minutes
Lead28-36 days
Mercury65 days
Methamphetamine6.4-15 hours
Methylphenidate2-3 hours
Plutonium40 years (liver), 100 years (bone)
Phenytoin7-42 hours
Tetrahydrocannabinol (cannabis/marijuana)Infrequent users: 1.3 days
Regular users: 13 days
Warfarin1 week

What is the formula for half-life?

The formula for half-life is: t1/2= [(0.693)*(Volume of distribution)]/Clearance or

t1/2= [(0.693)*(Vd)]/Cl

The half-life is directly proportional to the volume of drug distribution. It means the more the drug is distributed in the body, the more the half-life is. The half-life is inversely proportional to clearance, which indicates that the more the drug clearance from the body, the less the half-life is.

What are the main factors that affect drug half-life?

The two major factors that affect drug half-life include:

  1. Kinetics: Two types of elimination kinetics may affect the half-life:
    • First-order kinetics: The clearance rate directly depends on the initial concentration. The higher the amount, the higher the clearance. Most drugs follow first-order kinetics.
    • Zero-order kinetics: Drugs with zero-order kinetics are cleared from the body independent of their concentration. Alcohol is eliminated by zero-order kinetics. Some drugs that follow first-order kinetics switch to zero-order kinetics as saturation occurs with overdose.
  2. Age: The half-life of drugs increases with age. Because older people have more fat tissues than young people, there is a high distribution of drugs throughout the body. Furthermore, the metabolism of the drug may also decline with age. Due to longer half-lives, the elderly require fewer dosages of drugs than younger people.

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Medically Reviewed on 1/29/2021
References
Science Direct. Half Life Time. https://www.sciencedirect.com/topics/nursing-and-health-professions/half-life-time

Education, Science, Spirit of Sport. Half-Life of a Drug. March 12, 2015. USADA. https://www.usada.org/spirit-of-sport/education/drug-half-life/

Murphy M, Murray PT. Chapter 125: Critical Care Pharmacology. In: Hall JB, Schmidt GA, Kress JP, eds. Principles of Critical Care. 4th ed. McGraw Hill Education. https://accessmedicine.mhmedical.com/content.aspx?sectionid=80028183&bookid=1340