U01.05.005 Half-life (t1/2)

The half-life (t₁/₂) of a drug is defined as the time required to reduce the amount of the drug in the body by half during the process of elimination.

Importance in Drug Elimination

In pharmacokinetics, understanding half-life is essential for determining the duration of a drug’s effect and planning the dosing schedule. Half-life is particularly useful in:

Determining the dosing interval: A shorter half-life often requires more frequent dosing, while a longer half-life allows less frequent dosing.
Predicting the time to reach a steady state during constant infusion.

Half-Life in First-Order Kinetics

In first-order kinetics, the rate of elimination is proportional to the drug’s concentration. This type of elimination is typical for most drugs. Key points include:

Time to reach steady-state concentration: When a drug is infused at a constant rate, it takes 4–5 half-lives to reach a steady-state concentration in the body.
Time to reach 90% of steady state: A drug takes approximately 3.3 half-lives to reach 90% of its steady-state concentration.

Formula for Half-Life in First-Order Elimination

The half-life of a drug undergoing first-order elimination can be calculated using the following equation

$t_{1/2}=\frac{0.7\times V_d}{Cl}$

Where:

t₁/₂ = half-life
Vd = volume of distribution (how widely the drug is dispersed throughout the body)
Cl = clearance rate (how efficiently the drug is removed from the body)

Points to Remember

Half-life is a critical factor in determining how often a drug should be administered.
In first-order kinetics, it takes 4-5 half-lives to achieve a steady state.
Understanding half-life helps in predicting the drug’s behavior in the body.

Table: Key Concepts of Half-Life

Parameter	Description
Half-Life (t₁/₂)	Time required to reduce drug concentration by 50%.
Steady-State	Reached after 4-5 half-lives with constant drug infusion.
90% of Steady-State	Achieved after approximately 3.3 half-lives.
Formula	$t_{1/2}=\frac{0.7\times V_d}{Cl}$

Bibliography Reference Links

Rang HP, Dale MM, Ritter JM, Flower RJ, Henderson G. Rang & Dale’s Pharmacology. 8th ed. Elsevier; 2015.
Katzung BG, Masters SB, Trevor AJ. Basic and Clinical Pharmacology. 14th ed. McGraw-Hill Education; 2017.

USMLE Step 1 Review Series

Curriculum