Posted by Sunnely on April 5, 2002, at 17:09:09
In reply to FACHAD: 2 questions 4 U (1/2 lives revisited!), posted by Janelle on April 4, 2002, at 23:50:20
I'll do my best.
First, as a general rule, it takes 4-5 half-lives to eliminate greater than 90% of the amount of drug given. The drug is not completely "gone" from the body but at this point, no longer considered clinically significant.
If a drug's half-life is determined to be 2 hours, it doesn't matter whether the drug is given single doses of 100 mg or 200 mg, it will take approximately 4-5 half-lives to eliminate greater than 90% of the drug. (Provided there are no other interfering factors.)
For example, a drug with 2 hour half-life, given a single oral dose of 100 mg:
100 mg given at the onset
amount drops to estimated 50 mg after 2 hours (1 half-life)
amount drops to estimated 25 mg after 4 hours (2 half-lives)
amount drops to estimated 12.5 mg after 6 hours (3 half-lives)
amount drops to estimated 6.25 mg after 8 hours (4 half-lives)
amount drops to estimated 3.125 mg after 10 hours (5 half-lives)
So, based on this, for a drug with a half-life of 2 hours, given as a single oral dose of 100 mg, it will take about 8-10 hours to eliminate >90% (greater than 90%) of the drug given.
A drug with half-life of 2 hours given at a single oral dose of 200 mg:
200 mg at the onset
amount drops to estimated 100 mg after 2 hours (1 half-life)
amount drops to estimated 50 mg after 2 hours (2 half-lives)
amount drops to estimated 25 mg after 2 hours (3 half-lives)
amount drops to estimated 12.5 mg after 2 hours (4 half-lives)
amount drops to estimated 6.25 mg after 2 hours (5 half-lives)
Based on this, it will take about 8-10 hours to eliminate >90% of the drug with a half-life of 2 hours, given 200 mg single oral dose.
So, both drugs whether given at 100 mg and 200 mg single oral dose, with similar half-lives of 2 hours, will take 8-10 hours to be eliminated from body, >90% of the amount given.
In conclusion, as a general rule, no matter what the concentration of a drug is, it will take between 4-5 half-lives to eliminate >90% of the drug from the body, provided there are no other interfering factors.
NOTE: For some drugs, their half-lives change (or become more stabilized) after repeated doses.
> Hi there!
>
> 1) I'm afraid because I typed it all in caps and there is another thread right below it all in caps that my other question regarding half-lives might get missed, so could you please go back up to the HALF-LIFE thread I started and answer the question that begins with "RELATIONSHIP BETWEEN" ... you could answer it up there, that would be great, thanks!
>
> 2) Please humor me and bear with me a bit more here cuz I'm still not getting the whole story with this half-life stuff (no pun intended!) ...
>
> Okay, taking your example of a drug concentration of 100mg/l with a 2 hour half-life, the drug would be reduced as follows(?):
>
> 1PM 100 mg/l
> 3PM 50 mg/l
> 5PM 25 mg/l
> 7PM 12.5 mg/l
> 9PM 6.25 mg/l
> 11PM 3.125 mg/l
> 1 AM 1.56 mg/l (I suppose at this point the drug could be considered out of the body)
>
> Adding this up, I'm getting 6 half-lives, but you said that after FIVE half-lives, a drug is considered gone??
>
> ALSO, what confuses me even more is what if the concentration was doubled, and it was 200 mg/l of the drug with a 2 hour half life -- wouldn't it take MORE half-lives to get rid of the higher concentration of the drug?
>
> I would think that the higher the concentration of a drug, the more half-lives it would take to eliminate it???????
>
> So, I don't understand how no matter what the concentration of a drug, it is always 5 half lives to eliminate it from the body??????? How can this be? HELP!!!!!!!!
>
> Thanks!
poster:Sunnely
thread:101843
URL: http://www.dr-bob.org/babble/20020402/msgs/102040.html