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Re: Half Life of drugs

Posted by JD on May 4, 1999, at 12:30:46

In reply to Re: Half Life of drugs--SEAN, posted by RUTH on May 4, 1999, at 9:35:02


Half-lives are very complicated, as I learned myself by trying to find out about them on the internet! In general, as Sean says, the half-life is typically the amount of time it takes for the amount of a drug in your blood to drop to 50% of its peak level. In theory, this should mean that if you take 20mg of Celexa Monday morning that gets quickly absorbed into your bloodstream, half of it will be out of your bloodstream by 35 hours later. You're right to realize that drugs with longer half-lives tend to build up in your body--If you take Celexa every day for at least a week or so, the total amount in your bloodstream at any one time will include the fractional "left-overs" of all the previous days' doses, which would probably amount to between two and four times the amount of a single dose. (Depending on the length of the half-life, a drug should level out at what's called a 'steady state' after a certain amount of time on a fixed dosage).

Technically, if you stop "cold turkey" while you're at a steady state, it would take the same-old 35-hour half-life for your blood levels to go down to half of this steady-state level (which might still be more than the amount of a single isolated dose). On the other hand, to wait to get absolutely ALL of the drug out of your system would take a while, though -- Think of folding a piece of paper in half over and over again to get the general idea of what this entails... For your purposes (managing side-effects), however, you may find that even a one-day 'drug holiday' brings your levels down enough to make a big enough difference. (I know this has worked well with Zoloft and Paxil, but they have somewhat shorter half-lives, so perhaps you'd need to go with two days.) The general principle should be not to compromise the therapeutic effect of the med, of course.

I should say that all of the above is rather oversimplified, too. Different drugs act in the body in different ways. As you note, drugs can sometimes behave differently depending on the dosage and how frequently they're taken--I'm not quite sure where Celexa stands in regard to this issue, though it's probably not a huge deal. Perhaps more importantly, the level of a drug in your bloodstream may also not always correlate to its presence and effects elsewhere in your body. While a great many drugs follow the general half-life rule (a 50% drop in concentration over a given amount of time), the complexity of the body means that extrapolating to overall effects can be a very uncertain business! So in the end, you will likely have to do some careful experimenting to figure out what works best for you. (Best to discuss this with your doctor, obviously.)

FYI, it's not nutty at all that a drug with a half-life of well over a day might do just fine being taken less than 7 days a week. Prozac is perhaps the best example--it's half-life is so long (five to ten days, I think) that some doctors have even started prescribing it weekly for patients who don't need the big build-up that comes from taking a med like this every day.

Hope this has helped a bit. Good luck!





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