A useful measure of a drug's longevity in the body is the elimination
half life. This can apply only to drugs
with first order kinetics which is comparable to the pattern
of decay of radioactive materials. It is the time it takes for elimination of half of the
original amount or concentration of drug. For example, if the half life of a drug
is 24 hours, at the end of every day there will be half as much drug remaining as there
was at the beginning of the day. After about 5 half lives after cessation of
dosing the drug should be almost completely eliminated. After about 5 half lives
of regular dosing the concentration of the drug should reach a plateau.
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