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Drug Half-Life

The Half-life of a Drug 

The data we can go to is in most Pharmacology books and they will say something like this quote from Basic Clinical Pharmacology by Michael Winter: 

"The time required for a drug concentration to reach steady state is determined by the drug's half-life. It takes one half-life to reach 50%, 2 half-lives to reach 75%, 3 half-lives to reach 87.5%, 3.3 half-lives to reach 90%, and 4 half-lives to reach 93.75% of steady state....In most clinical situations, the attainment of steady state can be assumed after 3-5 half-lives." 

The same numbers 3-5 half-lives are used to calculate the elimination of the drug. 

If we take Klonopin's longest half-life number, which is 50 hours, that would be 50hours x 5 = 250 hours = 10.42 days. So it takes around 10 days for Klonopin to build to full strength is the blood stream and likewise it takes about that time for the drug to be eliminated. 

In the case of valium, the longest half-life is 200 hours. So full strength is achieved after 200hours X 5 = 1000hours = 41.66 days. 

Note that build up and elimination of all drugs can be calculated in the same way. 

It should also be noted that build up and elimination happen most at the beginning at less at the end of the time frame.


Disclaimer:  The information contained in this website was not compiled by a doctor or anyone with medical training. The advice contained herein should not be substituted for the advice of a physician who is well-informed in the subject matter discussed. Before making any decisions about your health or treatment you should always confer with your physician and it is always assumed that you will do so.

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Last updated 21 July 2020