Posted by Sunnely on June 29, 2001, at 23:00:59
In reply to Celexa half-life?, posted by Kingfish on June 28, 2001, at 14:12:16
The half-life of citalopram (Celexa) is approximately 35 hours. Half-life is the time during which half or 50% of an amount of drug is cleared from the blood, usually expressed in hours.
As a general rule, it would take approximately 4 to 5 half-lives for a drug to reach a steady-state blood level, provided all factors remain constant. (Steady-state blood level is the point at which tissue concentrations of a drug become stable that is, when the amount of the drug eliminated every 24 hours is equal to the amount administered every 24 hours, provided all factors remain constant.) Similarly, once steady-state blood level is reached and the drug is stopped, it would take approximately 4 to 5 half-lives for the drug to be eliminated from the system (at least 90% of the drug), provided all other factors remain constant.
If you have been taking Celexa regularly at the same dose of 40 mg/day for quite sometime, you would have been at steady-state level prior to discontinuing the drug. Based on the above general rule, it would take approximately 7 days to eliminate more than 90% of Celexa from your system.
Celexa is metabolized by 2 different types of liver or gut enzymes namely, CYP2C19 and CYP3A4. Fluoxetine (Prozac) is an inhibitor of a number of liver enzymes namely, CYP2D6 (markedly potent) and CYP2C9, CYP2C19, and CYP3A4 (all moderately potent). Had Prozac been not administered immediately after stopping Celexa, it would have been eliminated from your system (more than 90% of Celexa) in approximately 7 days. When Prozac 40 mg/day was administered right after stopping Celexa, Prozac inhibited the metabolism of Celexa via the above-mentioned gut enzymes, leading to prolonged half-life and delayed clearance of Celexa from your system. (Note: The addition of Prozac is the one that changed the "constant factor.") Since cytochrome inhibition from drug-drug interaction occurs immediately (as early as the first dose), a sudden rise in Celexa blood level occurs, leading to increase serotonin supply, leading to activation of serotonin receptors in the gut (among other things), leading to what you experienced, woozy and nauseous. (Note: The greater the dose of an inhibitor that is administered, within the range of clinically useful doses, the greater the extent of the inhibition that should occur. For example, Prozac, a competitive inhibitor of the above-mentioned liver or gut enzymes, should produce a greater inhibitory effect at a dose of 40 mg or 60 mg than at 20 mg/day.)
Fortunately, you did not experience serotonin syndrome which is a potential drug-drug interaction with the combined use of SSRIs.
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> Does anyone know how long Celexa stays in your system? My pdoc switched me directly from 40 mg Celexa to 40 mg Prozac with no interval/taper. I feel generally O.K. but a little "woozy" and nauseus.
>
> Thanks!
>
> - K.
poster:Sunnely
thread:68265
URL: http://www.dr-bob.org/babble/20010625/msgs/68428.html