Question 1

What is half-life in pharmacokinetics?

Accepted Answer

Half-life (t1/2) is the time required for the plasma concentration of a drug to decrease by 50%. It determines how frequently a drug must be dosed and how long it takes to reach steady state (approximately 4-5 half-lives).

Question 2

How do I calculate half-life from two concentrations?

Accepted Answer

Measure two plasma concentrations (C1 and C2) at times t1 and t2 during the terminal elimination phase. First calculate the elimination rate constant: Ke = ln(C1/C2) / (t2 - t1). Then t1/2 = 0.693 / Ke.

Question 3

What is a normal drug half-life?

Accepted Answer

Drug half-lives range from minutes (e.g., adenosine ~10 seconds) to weeks (e.g., amiodarone ~40-55 days). Most oral drugs have half-lives between 2-24 hours. A half-life >24 hours typically allows once-daily dosing.

Question 4

Why is half-life important for dosing?

Accepted Answer

Half-life determines dosing frequency, time to steady state, and drug accumulation. Drugs with short half-lives need frequent dosing, while long half-life drugs may need loading doses to achieve therapeutic levels quickly.

Question 5

What is the relationship between half-life, clearance, and volume of distribution?

Accepted Answer

Half-life = (0.693 x Vd) / CL. A drug can have a long half-life due to either low clearance (poor elimination) or high volume of distribution (extensive tissue binding), or both.

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