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Re: Half Life--JD... P.S.

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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