A patient reaches a steady state level of lithium that produces toxic side effects. If he decides to discontinue his medication, how long would it take for his lithium blood levels to reach 25% of his original steady state levels assuming a half life of 22 hours for lithium?

Lithium, like most drugs follows first-order kinetics, which means a constant percent of the drug is eliminated per unit time. His drug levels will decrease by 50% every half-life. Therefore, they will be 50% of original levels after one half-life, 25% after two half-lives, 12.5% after three half-lives, etc. Two half-lives is 22 x 2 = 44 hours. Ref: Buxton I.L., Benet L.Z. (2011). Chapter 2. Pharmacokinetics: The Dynamics of Drug Absorption, Distribution, Metabolism, and Elimination. In L.L. Brunton, B.A. Chabner, B.C. Knollmann (Eds), Goodman & Gilman's The Pharmacological Basis of Therapeutics, 12e.