Elimination after 4 half-lives in first-order kinetics is:

The Plasma half-life ( t1/2) of a drug is the time taken for its plasma concentration to be reduced to half of its original value 1 t 1/2-50% drug is eliminated. 2 t 1/2 -75% (50 + 25) drug is eliminated. 3 t 1/2 -87.5% (50+ 25 + 12.5) drug is eliminated 4 t 1/2 -93.75% (50 + 25 + 12.5 + 6.25) drug is eliminated In first-order kinetics-t 1/2 remains constant because Volume of distribution and Clearance do not change with dose. after 5-6 t1/2 drug is almost completely eliminated from the body Ref: KD Tripathi pharmacology 7th edition (page.no: 31)