half life

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.