Posted by Larry Hoover on April 20, 2003, at 13:31:11
In reply to HELP! Questions about med HALF-LIFE !, posted by Janelle on April 19, 2003, at 20:49:17
> 1. What is considered a short half-life, middle half-life and long half-life?
My thinking would put short half-life at less than 6 hours, and long at greater than 24 hours. The rest fall in the middle.
> 2. Is half-life dosage related, meaning if you're on a higher dose of a med, it will have a higher half-life (take longer to clear out of your system?) I think this is NOT the case, that the two are independent, but I cannot remember.
> Thanks to whoever can answer these.
The answer depends on the order of the reaction. Without getting into calculus (I don't think you want to go there, right?), there are three possible relationships.
1. A zero-order reactant is metabolized at a constant rate, regardless of the initial concentration. An example you're familiar with is alcohol. Typically, people metabolize alcohol at between one and one and half ounces per hour. The more you drink, the longer it takes.
2. A first order reaction proceeds independent of initial concentrations. By far, this is the most common sort we consider when we think of drugs and their half-lives. In this group, half-life is a fixed time (which varies between people, but is constant for each individual).
3. The third type is second order. It does depend on concentration, but in a bizarre way. The higher the initial concentration, the shorter the half life. Each subsequent half-life is twice as long as the one preceding it. I can't think of an example off the top of my head, but they do exist.
So, example two is probably the most relevant one to consider. In that example, dose has no effect on half-life.
Lar
poster:Larry Hoover
thread:220748
URL: http://www.dr-bob.org/babble/20030417/msgs/220868.html