Posted by medlib on April 20, 2003, at 16:36:16
In reply to HELP! Questions about med HALF-LIFE !, posted by Janelle on April 19, 2003, at 20:49:17
Hi Janelle--
Although a med's half-life is not affected by its initial concentration (dose amount times bioavailability), the level of med in your bloodstream at any given time *is* affected by initial dose amount.
Most medication is given in an amount and on a time schedule necessary to maintain a steady state concentration in the bloodstream within the med's therapeutic range. The therapeutic range of a drug is from the lowest dose which produces positive effects to the highest dose which doesn't produce toxic effects.
I'll use Seroquel, which has a 1/2 life of 6h and usually is given twice a day (bid), as an example. 6 hours after an initial dose of 200 mg.s, 100 mg.s would remain in the bloodstream. It might seem that if 1/2 is gone in 6h, all of it should be gone in 12h; but that is not the case. After 12 hours 50 mg.s remains (1/2 of the 1/2 left). At this point the second dose of 200 mg.s is taken (bid), making the blood concentration 250 mg.s. At the eighteenth hour the blood level is (roughly) 125 mg.s (1/2 of 250); and at 24 hours, it is 62.5 mg.s (1/2 of 125) and it is time for the next dose. In this manner, the blood level of Seroquel reaches a steady state in about 2 days. In actuality, it's not quite that simple, because one must consider the drug's time to peak concentration (Seroquel's is 1.5 hrs). I used Seroquel as an example because you had asked about it above and because its metabolites are not pharmacologically active (which would complicate calculations). Does this make any sense? If not, perhaps Larry will be willing to translate it into something more coherent.
Btw, Seroquel acts as an antihistamine, among other things; that may be what is making you sleepy. Some people adjust to it so that it's no longer soporific; but, as I tried to explain above, after a couple of days roughly the same level of Seroquel will be in your system at all times.
Hope this is of some help.---medlib
poster:medlib
thread:220748
URL: http://www.dr-bob.org/babble/20030417/msgs/220910.html