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Half-life over 24 hours and daily intake: does the drug accumulate

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can someone tell me what it means when a drug has a half life of say, 32 hrs?

does that mean it's still having an effect on you for at least 32 hrs after taking a dose?

if you take a dose in 24 hrs after the first one, does the effect accumulate, because the part of the first dose is still affecting you?



Edited by Altostrata
merged topics and retitled

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Great question catnapt


The half life is the period of time required for the amount [concentration] of drug in the body to be reduced by one-half.


if you take a dose in 24 hrs after the first one, does the effect accumulate, because the part of the first dose is still affecting you?

you're onto it catnapt!


I just looked up half-life of zyprexa and it says 30 hours. So i guess thats close to 32 so i assume you are referring to zyprexa's half life here.


What this means is that if one takes a 2.5 mg tablet then it will take  30 hours (1.5 days) (lets go with 30 hrs it makes things easy(er)...you'll see in a minute),  for the amount of zyprexa in the blood to fall to 1.25mg


However thats not what will be in the blood cos in the meantime you just swallowed another one.


I'm not an expert in this so dont hold me to anything here but lets have a look at whats going on  based on the defn of half life.

This is also slightly tricky cos we want to find out how much drops out in 24 hrs not 30 hrs so by ratio i would assume its  equivalent to a drop of 1mg after 24 hrs


So levels in your blood i assume go like this...

day 1

2.5mg                                         total in blood 2.5mg


day 2

2.5( another 2.5 is swallowed),  1.5  (day one's dose has now dropped 1mg to 1.5mg)            total 4.0 mg in blood


day 3

2.5,  1.5, 0.5  (day ones dose is now dropped to 0.5)                     total 4.5


2.5, 1.5, 0.5, -    (day ones dose has now gone)                                total 4.5mg



2.5, 1.5, 0.5, -                                                                                 total 4.5 mg




Note how after three days there is now a constant amount of 4.5mg in the blood. A steady state now exists. ie the introduction of the drug keeps pace with its removal.


This is why the moderators will tell you it takes about 4 days for the drug to reach a steady state in the blood.


I've made one or two wee assumptions here so it could be oversimplified....but i think this is basically it.




edit: I just posted and i see that Lex posted in the meantime.

Sorry for the doubleup.

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I was wondering, i dont know very much about half lifes of meds and steady states, etc. But i am curious on something, lets take zoloft for example, it has a half life of 24 to 26 hours. this would mean that if you take 50 mg today, tomorrow you would still have 25 mg in your blood, but tomorow you would take another 50 mg tablet, if the half life of the first pill is still there, it would mean that on the second day, we would have 75 mg of zoloft in our blood right? And then Since they are separate doses, they half lives would vanish indepedently, if so it would accumulate, i am missing something here, sorry for my curiosity., can someone explain how this reallly works?


According to what i understand this simple table shows that at the seventh day we would still have 98,8 mg on blood?  The numbers separated from / are the half lives of the pill from the day before.



DAY        1           2                                         3                                4                                                  5                                                        6                                                      7                              
DOSE    50     25/50 = 75 mg           12.5/25/50 = 87,5 mg    6.25/12.5/25/50 = 93,75 mg   3.2/6.25/12.5/25/50 = 93,95    1.2/3.25/6.25/12.5/25/50 = 98,2 mg   0.6/1.2/3.25/6.25/12.5/25/50 = 98,8 mg



So this would mean a dose of 50 every day would mean that a steady state is double the dose we take every day? If so that is way to huge. ????



Since i am only taking 2 mg a day, my steady state is in fact the double of it:


DAY           1         2         3            4                          5                                 6                                           7                              
dose           2     1/2   0.5/1/2   0.25/0.5/1/2  0.125/0.25/0.5/1/2    0.06/0.125/0.25/0.5/1/2    0.03/0.06/0.125/0.25/0.5/1/2 = 3.965 mg

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The body is continually metabolizing the medication, even after the first half-life has expired. The approved and published daily maximium dose for a medication takes into account the ability of the body to metabolize that medication so that repeated daily doses do not lead to toxicity. This area of knowledge is called "pharmacokinetics."  If other medications are taken during the day, the capacity to metabolize may be affected. This is why we urge people taking more than one drug to obtain and post an interactions report. Drugs-dot-com Drugs Interactions Checker.


If you want to delve into these questions further, a web search using keywords "drug name " and "pharmacokinetics "

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Merged similar topics. (Thanks for finding them, ChessieCat.)

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