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Medication Half-Life


Updated August 04, 2007

A medication's half-life is the time it takes for the plasma concentration of a drug to reach half of its original concentration. More simply put, the half-life of a drug is how long it takes for half of it to be eliminated from the bloodstream.

One impact of half-life is found in the SSRI antidepressants. People taking SSRIs with short half-lives are much more likely to experience SSRI Discontinuation Syndrome. People taking an SSRI with a long half-life such as Prozac need to wait far longer between stopping Prozac and starting to take an MAOI antidepressant (see MAOI Antidepressants).

Interestingly, no matter what the half-life of a medication is, it takes about 4 half-life units for the concentration of a medication in the system to reach a steady state. Thus if you begin taking a medication with a half-life of 24 hours, on the fifth day the rate of intake of the drug will approximately equal the rate of elimination. If the half-life is 12 hours, you'll reach that state at the beginning of the third day.
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