Time required for the concentration of a drug to decrease by one half For a first order reaction the half life is constant

Equation :

Apparent volume of distribution (Vd)

Volume of body fluid in which the drug is dissolved. 

Significance of Vd: 

To estimate the amount of drug in the body relative to the conc in the plasma.

Vd – Apparent volume of distribution

Cp – Plasma concentrationD_B – Amount of drug in the body

Vd after IV injection

D_B⁰ – Dose of drug given by IV bolus C_p⁰ – Extrapolated conc of drug at time= 0