Posted by viridis on February 8, 2003, at 0:04:19
In reply to Re: that doesn't look right either (arghh!) (nm), posted by Larry Hoover on February 7, 2003, at 15:51:34
Half-life is a somewhat confusing concept (I know, because I regularly have to explain half-lives of radioisotopes to students in terms of carbon dating of fossils etc.).
The basic idea (I'll leave out the math) is that one half-life for a med is the time it takes half the original amount of the drug to clear your system. So, many people would say that, if the half-life is 10 hours, then all of it should be gone in 20 hours. But it doesn't work that way -- instead, in the next 10 hours, half of what's remaining will be gone, then half of that in the next 10 hours etc. Thus, 50% is gone after 10 hours, 75% after 20 hours, 87.5% after 30 hours, and so on.
So, theoretically, it's never completely gone (although really it will be eventually, for all intents and purposes).
I can't address the specifics of certain meds, but in general, after several half-lives, there's a negligible amount left. Just as a reference point, for the radioisotopes of phosphorus that I work with (which are fairly potent but have short half-lives), they're considered safe to throw in the regular trash after 10 half-lives. This is an extremely conservative, safety-based approach, and at that point there's no more radioactive isotope left than what's found naturally in the environment.
poster:viridis
thread:139500
URL: http://www.dr-bob.org/babble/20030204/msgs/140089.html