<prompt> Steady state concentration of a drug is achieved in: September 20072-3 half lives3-4 half lives4-5 half lives5-6 half lives  <response>: Ans. C: 4-5 Half Lives When a drug is infused IV, the plasma concentration continues to rise until elimination equals the rate of delivery into the body. Regardless of the drug, 50 of the plateau concentration is attained in 1 half-life of the drug; for 2, 3, and 4 half-lives, 75, 87.5, and 93.75 of the plateau concentration are reached, respectively. For practical purposes, steady state is achieved by 4-5 half-lives. 