What Does Half Life Mean in Drugs? Formula & Factors

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.

Generic Name Half-life (T1/2) Alprazolam 6-12 hours Amiodarone 15-142 days Amphetamine 10-12 hours Atenolol 6-7 hours Clonazepam 18-50 hours Cocaine 50 minutes Diazepam 20-100 hours Donepezil 70 hours Dutasteride 5 weeks Erenumab 28 days Fluoxetine 2-4 days Heroin 2-6 minutes Lead 28-36 days Mercury 65 days Methamphetamine 6.4-15 hours Methylphenidate 2-3 hours Plutonium 40 years (liver), 100 years (bone) Phenytoin 7-42 hours Tetrahydrocannabinol (cannabis/marijuana) Infrequent users: 1.3 days

Regular users: 13 days

Warfarin 1 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:

• 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.
• 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.
• 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.

Top What Does Half Life Mean in Drugs? Related Articles

• Types of Diabetes Type 2 MedicationsConsumer information about the types of type 2 diabetes medications. Read about the side effects of Precose, Diabinese, Amaryl, Glucophage, Actos, Avandia, Starlix, Prandin, and more. Drug...
• Asthma MedicationsThere are two types of asthma medications: long-term control with anti-inflammatory drugs and quick relief from bronchodilators. Asthma medicines may be inhaled using a metered-dose inhaler or...
• Cold, Fever and Flu Symptoms in Children: Medications and Home RemediesHow long does a cold last? How long is a cold contagious? Colds and fevers are some of the most common ailments in children. Learn common cold symptoms, treatment options, over the counter (OTC)...
• Drug Interactions: What Foods, Drugs, Herbs Affect Medications?What foods, drugs, and herbal supplements interact with your pharmaceuticals? Learn about grapefruit and other common drug interactions to medications like warfarin, tramadol, Zoloft, trazodone,...
• What Can Cause a False Positive Drug Test?Getting ready to take a drug test for a job or for other reasons? Watch out for these common things that could lead to a false positive result.
• Medical MarijuanaMedical marijuana (medical cannabis) is a medicine that is plant-based. There are two species of medical marijuana; 1) Cannabis sativa, and 2) Cannabis indica. Medical marijuana is used to treat...
• Safe Cold Flu Drugs With DiabetesIf you have diabetes and catch a cold or the flu, it can be more difficult to recover from infections and their complications, for example, pneumonia. Home remedies and over-the-counter (OTC) drugs...