Posted by fachad on April 4, 2002, at 21:01:57
In reply to Could someone explain HALF-LIFE w/ example!, posted by Janelle on April 4, 2002, at 18:11:03
Ok, Janelle, here's my attempt:
Half-life is the time it takes for the concentration of a drug in your system to drop by 50%.
Here's a simple example. Suppose you have a concentration of 100mg/l (100 milligrams of drug per liter of blood) at say 1 PM, and the drug has a half-life of 2 hours. If you had blood drawn and tested every 2 hrs it would look like this:
1PM 100 mg/l
3PM 50 mg/l
5PM 25 mg/l
7PM 12.5 mg/l
Notice that in the first 2 hours, between 1PM and 3PM, your body got rid of 50mg/l of the drug.
But then in the same amount of time, the 2 hours between 3PM and 5PM, your body only got rid of 25 mg/l.
But in all cases, your body gets rid of half of the however much of the drug is there in the 2 hour interval. That is why it is said to have a 2-hour half-life.
The explanation for this is that the rate at which you body breaks down drugs (well, those with linear pharmacokinetics, anyway) is relative to the concentration of the drug in your system, not an absolute rate of elimination. (It get’s rid of 50% per 2 hours, not a fixed amount per 2 hours)
This is really significant because drugs with short half-lives will wear off quickly. This could be a good thing, like a sleeping pill that doesn't make you groggy the next day. But it could be bad if your symptoms break thru.
Also, if the half-life is short and you want to keep the concentrations in your system steady, you have to take the drug every half-life.
So if you want steady concentrations of a drug with a 24-hour half-life, you can take it once a day. If you want steady concentrations of a drug with an 8 hr half-life, you need to take it 3 times a day.
The problem with drugs with really long half-lives is that the build up in your system and can get to really high concentrations, even if you are only taking them once per day.
Prozac can still be in your system 6 weeks after you take the last dose.