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Old 03-20-2015, 01:13 PM
 
Location: Niagara Region
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I understand that the definition of 'half-life' is the amount of time it takes the body to get rid of half the dose, but I'm not clear on what that really means, for example, if the half-life of caffeine is 5.7 hours, does that mean it takes 11.4 hours to completely be eliminated from your system? Is there more to it than this? If there isn't, why is it important to define it that way rather than defining what the 'whole life' of a medication is?
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Old 03-20-2015, 01:35 PM
 
Location: in here, out there
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Just take the pills.
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Old 03-20-2015, 01:49 PM
 
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One study some years ago showed that the half-life of caffeine in healthy adults is 5.7 hours (see source). This means if you consume 200mg of caffeine at mid-day, you would still have 100mg in you at around 5.45pm.

So yeah, it would take twice as long to get rid of it.
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Old 03-20-2015, 02:23 PM
 
Location: Niagara Region
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Yes I read that too, so954. I was wondering what the significance or importance of the halfway point was, though.
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Old 03-20-2015, 03:20 PM
 
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Quote:
Originally Posted by Vectoris View Post
Yes I read that too, so954. I was wondering what the significance or importance of the halfway point was, though.

No you said you didn't know if the half life was 5 hours if it meant 2.5 hours for it to be half or 5 hours to be half or 10 hours to be out of your system.


The answer would be 5 hours is half the time.

The significance would be so you know when you need to take your next dose.

I would assume most medications are made to a certain strength that you only need 1 or 2 a day.
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Old 03-20-2015, 10:08 PM
 
1,037 posts, read 620,509 times
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Quote:
Originally Posted by Vectoris View Post
I understand that the definition of 'half-life' is the amount of time it takes the body to get rid of half the dose, but I'm not clear on what that really means, for example, if the half-life of caffeine is 5.7 hours, does that mean it takes 11.4 hours to completely be eliminated from your system? Is there more to it than this? If there isn't, why is it important to define it that way rather than defining what the 'whole life' of a medication is?
no, it means half is eliminated at 5.7 hours, half of whatever remains is eliminated at 11.4.

for example a 500mg dose of a drug with linear kinetics and a t 1/2 of 6 hours will have 250mg remaining at 6 hours, 125mg remaining at 12 hours, ~62mg remaining at 18 hours and 31mg remaining at 24 hours.

it helps to determine dosing intervals also taking into consideration also the amount needed in the body to have a therapeutic effect.

there is much more to it, pharmacokinetics can get complicated depending upon the pharmacokinetic properties of the drug.

this may help, on the drop down menu click on "the half-life"
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Old 03-21-2015, 12:17 AM
 
Location: Out there somewhere...
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Here's a good web site with clear explanation:
What You Should Know About Medicines/Half life, or how often to take it - Wikibooks, open books for an open world
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Old 03-21-2015, 06:57 AM
 
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[QUOTEso954;38893494]One study some years ago showed that the half-life of caffeine in healthy adults is 5.7 hours (see source). This means if you consume 200mg of caffeine at mid-day, you would still have 100mg in you at around 5.45pm.

So yeah, it would take twice as long to get rid of it.[/quote]

Not exactly, you would if you have 100 mg left after the first half life then you would have 50 left after the second half life, 25 after the third, and so on.
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