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Residual Medicine in the Body After n Hours or Days

Usually the medicine has a parameter called its half life which specifies the time for the residual medicine concentration to decay to half of its initial value.

If the medicine has a half life, `tau_(1/2)` then its exponential decay constant `tau_e` is

`tau_e=-tau_(1/2)/log_e(1/2)`

If the medicine is taken every day then the residual medicine in the body is

`"residual(t)"=sum_(i=0)^(i="floor(t)")exp(-(t-i)/tau_e)`      

where `"floor(t)"` is the integer part of the fractional time `t`. The sum is over `i` which is the integer day or hour number after the medicine is consumed. The plot below shows the residual medicine assuming a unit dose was taken every hour or day. The time required to obtain 99% of the final residual is

`t_(99%)=5tau_e`

`"residual(t)"=sum_(i=0)^(i="floor(t)")exp(-(t-i)/tau_e)`