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Drug therapy in chronic disease situations requires systemic drug levels to reach target steady-state levels for maximum safety and efficacy. The time it takes for a drug to reach steady-state is a function of the elimination half-life of the drug. The following table illustrates how long it will take to achieve steady-state relative to the half-life:

# of half-lives   % of Steady-State
150%
275%
387.5%
493.8%
596.9%

To achieve steady-state, you need approximately 5-7 half-lives of the drug. For drugs with rapid elimination and short half-life values, this is not a problem; however drugs with slow elimination could require days or weeks to achieve steady-state. If therapeutic effects are needed quickly, and the drug has a long half-life, one can use a loading dose to achieve therapeutic levels on the first dose. The loading dose rapidly achieves the therapeutic response and subsequent doses maintain the response.

$\textrm{Loading dose}=\frac{\textrm{Maintenance dose}}{(1-e^{-k\tau})}$