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Dr. Bob is Robert Hsiung, MD,
bob@dr-bob.orgShown: posts 1 to 5 of 5. This is the beginning of the thread.
Posted by Kingfish on June 28, 2001, at 14:12:16
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.
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.
Posted by Kingfish on June 30, 2001, at 10:20:27
In reply to Re: Celexa half-life? » Kingfish, posted by Sunnely on June 29, 2001, at 23:00:59
Yikes, Sunnely, was this not a good idea of my pdoc's then? Do I still have Celexa in my system? If so, how much longer do you think I will?
Thanks so much for your help!!
- K.
Posted by Sunnely on June 30, 2001, at 19:43:09
In reply to Re: Celexa half-life? » Sunnely, posted by Kingfish on June 30, 2001, at 10:20:27
If you are not getting any response from Celexa, nothing wrong with your doc's decision to switch from Celexa to Prozac. Although both drugs belong in the class of SSRIs, nonresponse to one SSRI does not confer similar response to another SSRI.
Other people may tolerate the immediate switch from Celexa to Prozac, while others such as you, may not. Different people respond differently to different drugs, therapeutic and side effects. (Did I say it right?)
I don't know if you still have Celexa in your system. One thing I know is, as long as you are no longer taking Celexa, even though you are still taking Prozac, your blood level of Celexa drops everyday until it is no longer in your system. Its clearance from your system just got delayed with the addition of Prozac, that's all. A lot of times this does not pose a serious problem.
Just be careful when taking other meds or herbs with Prozac. Prozac is a potent inhibitor of one gut enzyme and a moderate inhibitor of a couple of other enzymes. Prozac can raise the blood level of other drugs, if they happen to depend on these enzymes for metabolism. Always discuss with your doctor or pharmacist potential drug-drug interactions before taking any medication or herb.
***************************************
> Yikes, Sunnely, was this not a good idea of my pdoc's then? Do I still have Celexa in my system? If so, how much longer do you think I will?
>
> Thanks so much for your help!!
>
> - K.
Posted by Kingfish on July 2, 2001, at 10:08:41
In reply to Re: Celexa half-life? » Kingfish, posted by Sunnely on June 30, 2001, at 19:43:09
> If you are not getting any response from Celexa, nothing wrong with your doc's decision to switch from Celexa to Prozac. Although both drugs belong in the class of SSRIs, nonresponse to one SSRI does not confer similar response to another SSRI.
>
> Other people may tolerate the immediate switch from Celexa to Prozac, while others such as you, may not. Different people respond differently to different drugs, therapeutic and side effects. (Did I say it right?)
>
> I don't know if you still have Celexa in your system. One thing I know is, as long as you are no longer taking Celexa, even though you are still taking Prozac, your blood level of Celexa drops everyday until it is no longer in your system. Its clearance from your system just got delayed with the addition of Prozac, that's all. A lot of times this does not pose a serious problem.
>
> Just be careful when taking other meds or herbs with Prozac. Prozac is a potent inhibitor of one gut enzyme and a moderate inhibitor of a couple of other enzymes. Prozac can raise the blood level of other drugs, if they happen to depend on these enzymes for metabolism. Always discuss with your doctor or pharmacist potential drug-drug interactions before taking any medication or herb.
>
> ***************************************
>
> > Yikes, Sunnely, was this not a good idea of my pdoc's then? Do I still have Celexa in my system? If so, how much longer do you think I will?
> >
> > Thanks so much for your help!!
> >
> > - K.
This is the end of the thread.
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