The Halflife 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 halflife. It takes one halflife to reach 50%, 2 halflives to reach 75%, 3 halflives to reach 87.5%, 3.3 halflives to reach 90%, and 4 halflives to reach 93.75% of steady state....In most clinical situations, the attainment of steady state can be assumed after 35 halflives." The same numbers 35 halflives are used to calculate the elimination of the drug. If we take Klonopin's longest halflife 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 halflife 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.

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