MEDICATION HALF LIFE
Richard Walloch Ph.D.
The body excretes various medications at vastly different rates.
While some medications are virtually eliminated from the body
before the next dose is taken, other medications linger and are
actually built up with each dose. This build up, when it occurs, is
usually self limiting. A more or less stable level is maintained in
the body after a number of days or weeks.
Most medications are excreted by either the liver or the kidneys.
Some medications are modified within the body. A convenient
number to know is the medication's plasma half-life, which is the
amount of time it takes for the concentration in the blood to fall
Most medications have a short half life. The following graph shows
the theoretical plasma concentration of a medication with a half life
of 2 hours. The medication in this example is taken twice a day. The
concentration falls to near zero before each dose, and consequently
it does not significantly rise from day to day.
The picture is very different in the second graph where the half-life is long.
This graph shows the theoretical concentration of a medication with a half
life of 36 hours. Again, the medication is taken twice a day. The
concentration only falls about 25% after the first dose, and consequently it
rises with repeated doses over the following seven or eight days. Although
the body still eliminates only about 25% of the medication in each dosing
interval, the actual amount excreted increases as the plasma level increases.
Soon the amount excreted equals the amount of medication being taken,
and the plasma concentration becomes stable.
The tic marks on both two graphs represent days.
Long half lives is one of the reasons that some medications seem to
“kick in” only after a week or more. The concentrations of those
medications are actually building up during that week. But there
are other reasons for a delayed response to a medication. An antibiotic
may need time to combat an infection, or the body may need time to
use the medication in replenishing a component.
This little discussion should not be used by itself to adjust any medication. No particular medications
are plotted in the graphs, although there are medications with half lives in the ranges chosen. This
discussion is merely an attempt to illustrate how some medications can build up in our bodies, even
though we take the same amount every day.
Finally, we must remember that everyone is different. The published half life is the half life for the
average patient. Individuals will clear the medication either faster or slower than this published