What Does Half Life Mean in Drugs?
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.
- 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.