NAPLEX (North American Pharmacist Licensure Examination) Practice Exam

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How is the half-life (t1/2) of a drug calculated?

C2 = C1 x e^-kt
Ke = ln(C1/C2)/t
0.693/ke
(Desired concentration x Vd) / F

The correct answer is: 0.693/ke

The half-life (t1/2) of a drug is calculated using the formula: t1/2 = 0.693/ke. This formula is derived from the first-order elimination kinetic equation, where ke is the elimination rate constant of the drug. The half-life of a drug is the time required for the concentration of the drug in the body to decrease by half. Therefore, the correct formula to calculate the half-life of a drug is 0.693 divided by the elimination rate constant (ke). Option A (C2 = C1 x e^-kt) represents a first-order kinetic equation, but it is not the formula specifically used to calculate half-life. Option B (Ke = ln(C1/C2)/t) is not the correct formula for calculating half-life as it involves the elimination rate constant without considering the 0.693 factor. Option D ((Desired concentration x Vd) / F) is not used to calculate the half-life of a drug, but rather is related to dosing calculations. Therefore, the correct formula to calculate the half-life of a drug is 0.693 divided by the elimination rate constant (ke), as shown in option C.