Overview :

The half-life of a drug is the time needed for the concentration of the drug to decrease by half.

T(half) =

= (0.693 / (elimination rate constant)) =

= (0.693 * (volume of distribution) / (clearance))

Half-life is determined by both clearance and the volume of distribution:

(1) The half-life increases with reduced clearance.

(2) The half-life decreases with increased clearance (for example, with enzyme induction).

Isselbacher KJ,  et al (editors). Harrison's Principles of Internal Medicine, Ninth Edition. McGraw-Hill Book Company. New York. 1980. page 376.

Oates JA, Wilkinson GR. Chapter 66: Principles of drug therapy. pages 393-405. IN: Isselbacher KJ, Braunwald E, et al. Harrison's Principles of Internal Medicine, Thirteenth Edition. McGraw-Hill. 1994.

Rowland M, Tozer TN. Clinical Pharmacokinetics - Concepts and Applications. Third Edition. Williams and Wilkins. 1995. pages 19-20.