Welcome all to IS PHARMACOLOGY DIFFICULT Podcast! I am Dr Radhika Vijay, in today's episode, I will be dedication my talks to exclusively plasma half life, abbreviated as t1/2, its defined as time taken by drug to reach half of its initial concentration. First case to talk about today is a drug with one compartment distribution and first order elimination, in this case the semilog plasma concentration time plot has two slopes, the initial sharply declining alpha phase which refers to distribution, and later less declining beta phase which refers to elimination. It is the beta slope which we refer to half life of a drug. In reality most drugs have multi compartment distribution and multi exponential decay, so its the beta slope which we call half life, generally ignoring alpha phase, "k" is elimination rate constant of a drug which is derived from CL(clearance) divided by V (volume of distribution), in another equation, t1/2 equals to 0.693 multiplied by volume of distribution divided by clearance. Now important point to be noted is that in first order kinetics, volume of distribution and clearance do not change with dose, so t1/2 remains constant, while its not so in zero order kinetics. So all these important implications make lot of sense while evaluating pharmacokinetic parameters, Then I will wind up today's episode with examples of few significant half lives of some important clinically relevant drugs, till next episode, do follow me here, do rate and review on ITunes, Apple podcast, thank you!!