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 by 50%.

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.

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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.

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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 avarage.