Steady state describes a dynamic equilibrium where the drug's administration and elimination rates are equal. As a result, the drug concentration, called steady-state concentration or Css, stays constant over time, producing the desired clinical effect. Most dosage regimens include multiple doses repeated at fixed intervals or continuous IV infusions to achieve Css. The rate at which a drug's plasma concentration rises to attain the steady state depends on the drug's half-life, which is the time required to clear 50% of the drug from the body. When a continuous IV infusion starts, the drug's plasma concentration rises such that it takes one half-life to reach 50% of Css, two half-lives to be at 75% and nearly three half-lives to touch 90%. So, most drugs attain a steady state within four-five half-lives. All dosing regimens tend to maintain a drug's steady-state concentration to produce the most effective therapeutic response and least toxicity.