The percentage of drug remaining in the plasma after 3 half life is

HALF-LIFE of drug : * The time taken for its plasma concentration to be reduced to half of its original value 1st t1/2 50% 2nd t1/2 (50+25) 75% 3rd t1/2 (50+25+12.5) 87.5% 4th t1/2 (50+25+12.5+6.25) 93.75% So Remaining is 100 - 87.5% = 12. 5% Note : Nearly complete drug elimination occurs in 4-5 half lives. Drugs eliminated by First order kinetics Zero order kinetics t 1/2 remains constant because V and CL do not change with dose. t 1/2 increases with dose CL progressively decreases as dose is increased t1/2= 0.693 x Vd/ CL