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Dr. Bob is Robert Hsiung, MD,
bob@dr-bob.orgShown: posts 1 to 8 of 8. This is the beginning of the thread.
Posted by ruth on May 3, 1999, at 7:53:00
Can anyone explain to me in layman's terms
how the half life of drug works? here's a
practical example for you: I take 10 mg's of
celexa a day--I know the half life is 35 hours. How
long would it take for it to be out of my system
completely? Half of that? (like 18 hours?)
Does it depend on how long you take it?I seem to be able to get away with taking the lowest
dose of celexa a few days a week (4 times a week) and
still have a therapeutic effect. Maybe it's b/c my
depression is mild, I don't know.
Posted by Sean on May 3, 1999, at 12:05:48
In reply to Half Life of drugs--what does it mean?, posted by ruth on May 3, 1999, at 7:53:00
> Can anyone explain to me in layman's terms
> how the half life of drug works? here's a
> practical example for you: I take 10 mg's of
> celexa a day--I know the half life is 35 hours. How
> long would it take for it to be out of my system
> completely? Half of that? (like 18 hours?)
> Does it depend on how long you take it?
>
> I seem to be able to get away with taking the lowest
> dose of celexa a few days a week (4 times a week) and
> still have a therapeutic effect. Maybe it's b/c my
> depression is mild, I don't know.I believe half-life refers to a serum concentration
of half the peak level of a given dose. That is,
the *time* it take for a person's metabolism to
process the drug to half the peak level.Of course everybody is different and the actual
time to process a given medication differs greatly,
so the published half-life is an estimate, not a
specific prediction.Meds with longer half lives can have less frequent
dosing. But they also take longer to "wash-out"
completely...Does this help?
Sean.
Posted by RUTH on May 4, 1999, at 9:35:02
In reply to Re: Half Life of drugs--what does it mean?, posted by Sean on May 3, 1999, at 12:05:48
Sean,
Well...I'm still kind of confused, actually.
Maybe I phrased my question wrong--I was interested
in how long it takes for 10 mg's of celexa to
wash out of my system. They say the half
life of the normal dose (20 mg's) is 35 hours.
I was also wondering if
the longer you're on the drug, the longer it takes
to get out of your system--like does it build up
in your system...?..and in practical terms, I
guess what I was trying to accomplish was taking
weekend vacations from the drug so I wouldn't have
the sex side effects. So I was trying to figure
out when would be a good time during the week to
go off in order to be drug free by the weekend...
does that make sense?
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
Ruth,
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!
Best,
JD
Posted by ruth on May 4, 1999, at 14:46:00
In reply to Re: Half Life of drugs, posted by ruth on May 4, 1999, at 14:42:33
Thanks JD--I feel like I've had a very
thorough, complete and interesting inservice on
half lives! One more question for you though--
if the half life of 20 mg's of celexa is 35 hours,
would the half life of 10 mg's be 17 hours?
Posted by JD on May 5, 1999, at 5:36:36
In reply to Re: Half Life--JD, posted by ruth on May 4, 1999, at 14:46:00
That's just the thing--without getting technical, in most antidepressants a half-life is a half-life regardless of the initial amount; your body just gets rid of half that amount within the set time period. (On the contrary, a good example of a drug that's NOT like this is alcohol...) So the half-life of 10mg of Celexa would likely be around 35 hours too (i.e., about 5mg worth of it would still be in your system after 35 hours). Another way of looking at how this works is that your body basically gets rid of the drug at a rate that's proportional to how much is in your body at a given time, so the size of the dose doesn't make much of a difference for the half-life except in particular cases (like say massive doses). By the way, your mention of half-doses might be an interesting way to approach working out your situation, too.
Hope this helps. --JD
> Thanks JD--I feel like I've had a very
> thorough, complete and interesting inservice on
> half lives! One more question for you though--
> if the half life of 20 mg's of celexa is 35 hours,
> would the half life of 10 mg's be 17 hours?
Posted by JD on May 5, 1999, at 5:49:10
In reply to Re: Half Life--JD, posted by JD on May 5, 1999, at 5:36:36
I'll throw in one final thing based on your question, just cause this can be so confusing-- As I said, If you took 10mg Celexa, you could probably expect half of it to still be around in 35 hours. But since the 10mg dose would probably be on top of the past days' Celexa that was still left in your system, you'd also have 50% of that amount left after 35 hours too. The whole thing can be like one big, nightmarish math problem! ;-)
- JD
> That's just the thing--without getting technical, in most antidepressants a half-life is a half-life regardless of the initial amount; your body just gets rid of half that amount within the set time period. (On the contrary, a good example of a drug that's NOT like this is alcohol...) So the half-life of 10mg of Celexa would likely be around 35 hours too (i.e., about 5mg worth of it would still be in your system after 35 hours). Another way of looking at how this works is that your body basically gets rid of the drug at a rate that's proportional to how much is in your body at a given time, so the size of the dose doesn't make much of a difference for the half-life except in particular cases (like say massive doses). By the way, your mention of half-doses might be an interesting way to approach working out your situation, too.
>
> Hope this helps. --JD
>
> > Thanks JD--I feel like I've had a very
> > thorough, complete and interesting inservice on
> > half lives! One more question for you though--
> > if the half life of 20 mg's of celexa is 35 hours,
> > would the half life of 10 mg's be 17 hours?
Posted by pandey_m on May 9, 1999, at 20:51:20
In reply to Re: Half Life--JD... P.S., posted by JD on May 5, 1999, at 5:49:10
Here is a rule of thumb I worked out which may be of some use.Say you are taking m mg of some medication per day
for quite some time, and say its half-life is h days. Now if you stop the medication, the amount of medication in your system is roughly
1.5hm mg, ie one-and-half times the half-life multiplied by the daily dose.example - say you have taking 10mg of *any_med* twice a day, and say *any_med* has a half-life of 40 hours. So you are taking *any_med* 20mg/day, and its half-life is roughly 2 days, so on the day you stop you have 1.5 x 20 x 2 = 60mg *any_med* in your system.
After 40 hrs (the half life) your system has 60/2=30mg, after 80 hrs -- 30/2=15mg, after 120 hrs --- 15/2=7.5mg, after 160 hrs --- 7.5/2=3.25mg and so on ... after 280 hrs you have only 0.41mg and effective *any_med* has been flushed out of your system.Remember : you must use the longest half-life among those of the medication and its active metabolites if any.
[technical details :
assumptions : exponential decay; and half-life not much less than a day - in which case there is nothing to worry about really; and my "1.5h" rule gives a conservative estimate.
Sum the infinite series, drop higher order terms, reconvert to infinite series and drop higher order terms again. I tried computer simulations and it works reasonably well. Textbook formulas by contrast are long and unwieldy.
Of course a *day* is any period of time, only the same *day* should be used in "dosage/*day*" and "half-life in *day*s]
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