Pharmacology & Dosage
Intermediate

Drug Half-Life

Time for the plasma concentration of a drug to fall to half its initial value.

Formula

t1/2=0.693ket_{1/2} = \frac{0.693}{k_e}

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Variables

t_{1/2}Half-life
k_eElimination rate constant
0.693ln 2

Example

kₑ = 0.1/hr → t½ = 6.93 hours

Did You Know?

After five half-lives, about 97 % of a drug has been eliminated — this '5 half-lives' rule guides how long drugs stay active.

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