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Biological Half-Life – Definition and Clinical Relevance

The biological half-life indicates how long the body needs to eliminate half of a substance. It is a key measure in pharmacology and toxicology.

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Things worth knowing about "Biological Half-Life"

The biological half-life indicates how long the body needs to eliminate half of a substance. It is a key measure in pharmacology and toxicology.

What Is the Biological Half-Life?

The biological half-life (also referred to as the elimination half-life or t½) is a pharmacological term that describes how long the human body takes to reduce the concentration of a substance – such as a drug, chemical, or radioactive element – to half of its original value. This reduction occurs through the body's own processes, including metabolism, excretion via the kidneys or liver, and chemical transformation.

The biological half-life is a fundamental concept in pharmacology, toxicology, and medicine. It determines how often a medication must be taken and how long an active substance remains effective in the body.

Calculation and Units

The biological half-life is expressed in units of time – minutes, hours, days, or even years, depending on the substance. It is commonly denoted by the symbol t½. The formula is:

t½ = (ln 2 × Volume of Distribution) / Clearance
Simplified: t½ = 0.693 × Vd / Cl

Here, Vd stands for the volume of distribution (how widely the substance spreads throughout the body) and Cl for clearance (how quickly the body removes the substance).

Difference Between Physical, Biological, and Effective Half-Life

Especially in the context of radioactive substances, it is important to distinguish between different types of half-life:

Physical half-life: Describes the radioactive decay of an isotope, independent of the body.
Biological half-life: Describes the elimination of a substance by the body.
Effective half-life: Combines the physical and biological half-lives and indicates how quickly the total activity of a radioactive substance in the body decreases.

Factors Affecting the Biological Half-Life

The biological half-life is not a fixed constant – it can be influenced by numerous factors:

Age: In older adults, kidney and liver function are often reduced, which prolongs the half-life.
Body weight and composition: Fat-soluble substances accumulate in fatty tissue and are metabolized more slowly.
Liver and kidney function: Diseases of these organs significantly slow down the breakdown of substances.
Genetic factors: Differences in enzyme activity (e.g., CYP450 enzymes) affect the rate of metabolism.
Drug interactions: Certain medications can accelerate or slow the breakdown of other substances (enzyme induction or inhibition).
Urine pH: Affects the renal excretion of certain substances.

Clinical Relevance

The biological half-life has significant practical importance in medical treatment:

Dosing intervals: A medication with a short half-life (e.g., a few hours) needs to be taken more frequently than one with a long half-life (e.g., several days).
Steady state: After approximately 4–5 half-lives, a steady state is reached in the body, where the rate of drug intake equals the rate of elimination.
Washout period: After discontinuing a medication, it also takes approximately 4–5 half-lives for the substance to be almost completely eliminated from the body.
Toxicology: In cases of poisoning, the biological half-life provides information on how long a toxic substance remains in the body.

Practical Examples

The biological half-life varies considerably between substances:

Aspirin (acetylsalicylic acid): approx. 15–20 minutes (active metabolite salicylate: several hours)
Ibuprofen: approx. 2 hours
Diazepam (sedative): approx. 20–100 hours
Levothyroxine (thyroid hormone): approx. 6–7 days
Amiodarone (cardiac medication): up to 100 days
Lead in bone tissue: several decades

References

Brunton L., Knollmann B. – Goodman & Gilman's The Pharmacological Basis of Therapeutics. McGraw-Hill, 14th Edition (2022).
Rang H.P., Ritter J.M., Flower R.J. et al. – Rang & Dale's Pharmacology. Elsevier, 9th Edition (2019).
Rowland M., Tozer T.N. – Clinical Pharmacokinetics and Pharmacodynamics: Concepts and Applications. Lippincott Williams & Wilkins, 4th Edition (2010).

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