what does the term "half-life" mean with respect to drugs?

[Nev]

the only context i know of that term in is radioactivity.

i read something that said that it's the amount of time required to reduce the concentration in the body by one-half.

does that mean if you take a drug you will always have some of it in your system?

any chemists here?

[SineSwiper]

I think it's because some drugs don't decay at a steady rate. It doesn't exactly mean that the drug is going to linger on forever, but a half-life would be a more accurate figure of what a normal decay rate would look like. You could have your drugs that decay quickly within the first few hours and go steady afterwords, which would have a quick half-life. And then you could have the opposite, that have a slow decay but drop off quickly after a while, which would have a slow half-life.

Note: I'm not a chemist. I just like graphs.

[SineSwiper]

There's also a LD50 figure that is the lethal dose that kills 50% of test subjects (usually rats or real-life case studies).

[Gentz]

http://www-users.med.cornell.edu/~spon/ ... lfcalc.htm

[Sephy]

Half Life and drug elimination are sort of the same.
If the body didn't shuttle metabolites of the drug out of the system, yes, this would be the case. But it isn't. After about 5 half lives, the concentration of most drugs is appreciably small. Some drugs operate differently. The body hydrolizes the drugs, and moves them out through urine or other measures.

Chemistry student, anyway. Graduate in the fall.

[Nev]

so if you take a drug, will some of it always be in you then, by definition?

of course, food isn't really any different.

[Sephy]

No. The body will shuttle out any byproducts it doesn't uses or has too much of. If the body didn't have this mechanism, then yes, it would. Semantics...

[Nev]

i'm still confused.

so the half-life is the time it takes for the amount of "drug" remaining in your bloodstream to halve.

but the body will shuttle it out through other means?

you'll have to forgive me, i'm coming at this from the standpoint of my mathematics background, and in math, a half-life means the time it takes for something to halve. when you keep cutting something in half, it never hits zero.

[Gentz]

Uh, Mental, just because it's called a "half-life" doesn't mean that the rate of decay itself is one half. When you take a drug that drug is distributed through your blood in a certain concentration. Eventually, as your body metabolizes the drug, that concentration will be reduced to zero. Half-life is simply the amount of time it takes for the distribution of that drug in your blood to drop from peak concentration to one half of the peak concentration. I think the rate itself is quite linear - no asymptotes necessary.

[Nev]

oh!

got it

odd though...

why wouldn't they just say "time out of system" or something and just double the time?

[Gentz]

why wouldn't they just say "time out of system" or something and just double the time?

Yeah, I was wondering about that too. My guess is that it's probably hard to determine exactly when a drug is "completely out of your system" since the presence of a drug in your blood probably falls below our capacity to measure at a certain point, so we use an equation to determine it based on the amount of drug administered and the rate of decay/metabolization/removal.

[Nev]

hm, still weird to me...i still suspect...oh well though, i can be paranoid. something to think about for me vis a vis drugs anyway