Kaplan USMLE Step 1 prep: What’s half-life of investigational drug?


If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.  

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During an investigational study, the pharmacokinetics of a newly synthesized drug are studied in healthy volunteers. The volume of distribution and clearance determined in the first volunteer are 40 L and 2.0 L/hour, respectively.

Which of the following is the most likely half-life of the drug in this volunteer?

A. 2 hours.

B. 6 hours.

C. 14 hours.

D. 20 hours.

E. 28 hours.










The correct answer is C.

Graphic illustrating constant fraction for half life of a drug

The half-life (t1/2) of a drug is the time required for the amount of drug in the body or blood to fall by 50%. It is only applicable to drugs that exhibit first-order kinetics, in which a constant fraction of drug is eliminated per unit time as shown below. In zero-order kinetics, a constant amount of drug is eliminated per unit time.

T1/2 can be determined if the clearance (Cl) and volume of distribution (Vd) is known. Cl is the ratio of the rate of elimination of a drug to the concentration in the plasma (rate of elimination/plasma drug concentration). The Vd is the ratio of the amount of drug in the body to the drug concentration in the plasma (amount of drug in body/plasma drug concentration).

The half-life of a drug can be determined using the following equation:

t1/2 = (0.7 times Vd) / Cl

Therefore, t1/2 = (0.7 times 40L) / 2.0 L/hour, and t1/2 = 14 hours.

Note: 0.7 is a commonly used log approximation, but not the actual value. Another commonly used approximation is 0.693 for -ln(0.5) = 0.69315.

Graphic illustrating drug reaching steady state after four to five half lives

The half-life determines the rate at which a drug concentration rises during a constant infusion and also the rate at which the concentration falls after drug administration is stopped. It is commonly accepted that it takes four to five half-lives to reach steady state, as shown in the figure to the right.

  • The half-life (t1/2) is the time it takes for the plasma concentration of a drug or the amount of drug in the body to be reduced by 50%. 
  • The half-life of a drug can be determined using the following equation: t1/2 = (0.7 x Vd) / Cl, where Vd is volume of distribution and Cl is clearance. 

For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.

The AMA and Kaplan have teamed up to support you in reaching your goal of passing the USMLE® or COMLEX-USA®. If you're looking for additional resources, Kaplan provides free access to tools for pre-clinical studies, including Kaplan’s Lecture Notes series, Integrated Vignettes, Shelf Prep and more.