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Misc Half life question?

M

Mr Dobalina

Ex-Bluelighter
Joined
Jun 24, 2017
Messages
93
I dont really understand how you work out a half life of a drug.

If a drugs half life is 29 hours how many hours is it until the drug has cleared my system?

Is 27 hours correct?
 
Most half-lives for drugs refer to plasma half-lives which is the amount of time half of the drug is eliminated from your bloodstream. To figure out how long it will take to completely eliminate a drug from your system, you can multiply the half life by five. So if you take a drug that has a 29 hour half life, it'll take about 6 days to eliminate from your system.

However, you should keep in mind that this can vary quite a bit between individuals, so the numbers can be shorter or longer.
 
Slow_Mobius said:
Most half-lives for drugs refer to plasma half-lives which is the amount of time half of the drug is eliminated from your bloodstream. To figure out how long it will take to completely eliminate a drug from your system, you can multiply the half life by five. So if you take a drug that has a 29 hour half life, it'll take about 6 days to eliminate from your system.

However, you should keep in mind that this can vary quite a bit between individuals, so the numbers can be shorter or longer.
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I found a half life drug checker & it says for a 29 hour half life drug to be out of your system will take 13 days.
 
Most test books will say 4-7 half lives to completely eliminate a drug. After 5 half lives, you've eliminated about 97% if my mental math is right. So after six days, only about 3% of your initial dose will be in your blood stream per se.
 
Slow_Mobius said:
Most test books will say 4-7 half lives to completely eliminate a drug. After 5 half lives, you've eliminated about 97% if my mental math is right. So after six days, only about 3% of your initial dose will be in your blood stream per se.
Click to expand...
How come the half life drug checker says it takes 13 days to be out of your system?
 
"Out of your system" is also very relative. Technically by using half-life, it never could (Zeno/Xeno and all that). How long would it take for one last protein-bound molecule to get peed or oxidized away? How many loops around your body could it take? By math it never actually reaches zero.

So consider what it means to be "cleared". No longer detectable on a test? Which test? No more metabolic products or effects? Is it safe to take a reactive med now? Plus nearly everything has metabolites, and metabolites of those, and each of these has their own half-life, and oftentimes there's more than one path something can take; that will then depend on your personal genetics and other drugs you've taken and even what you've eaten.

Those are all reasons why there's no good answer, or at least it will depend too much, and part of the reason half-lifes are used (the other being how you measure kinetics).
 
Scrofula said:
"Out of your system" is also very relative. Technically by using half-life, it never could (Zeno/Xeno and all that). How long would it take for one last protein-bound molecule to get peed or oxidized away? How many loops around your body could it take? By math it never actually reaches zero.

So consider what it means to be "cleared". No longer detectable on a test? Which test? No more metabolic products or effects? Is it safe to take a reactive med now? Plus nearly everything has metabolites, and metabolites of those, and each of these has their own half-life, and oftentimes there's more than one path something can take; that will then depend on your personal genetics and other drugs you've taken and even what you've eaten.

Those are all reasons why there's no good answer, or at least it will depend too much, and part of the reason half-lifes are used (the other being how you measure kinetics).
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There is a good answer. It's about 5 half lives. That's what most pharmacology texts will tell you, and it accounts for things like protein binding. There eventually comes a point where the amount in circulation is negligible.

Discussions about detection periods are against the rules and are rarely the same as the amount of time it takes to eliminate a drug.

Mr Dobalina said:
How come the half life drug checker says it takes 13 days to be out of your system?
Click to expand...

It's just an extremely conservative estimate. Not everyone eliminates drugs at the same rate.
 
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Didn't mean to appear to disagree with you Mobius. They were just examples of why some citations vary.
 
Tolerance can last longer than a set # of half lives. abstaining can be important for a variety of reasons.
 
Scrofula said:
Didn't mean to appear to disagree with you Mobius. They were just examples of why some citations vary.
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No worries. Sorry I was curt with my response. And you're right that there can be some variation for a variety of reasons.

Captain.Heroin said:
Tolerance can last longer than a set # of half lives. abstaining can be important for a variety of reasons.
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Also a good point. Half-lives are useful for avoiding steady-state accumulation, but waiting 5 half-lives between doses doesn't necessarily mean you'll avoid addiction or tolerance issues. I think a lot of GABAergic drugs are notorious for having that problem.
 
I still dont understand. So the half life drug checker is wrong?
How do i work out when a half life of 29 hours is done?
29×5?
 
Yes, 29x5=145hrs, so six days, is what Slow_Mobius is saying. Scrofula and Slow also point out that these are all general parameters, that the drug check you've found is working at one end of of the possibilities of those parameters, and how for most practical purposes a drug has almost entirely worked its way out of your system once it has completed five half-life cycles. In the case of something with a 29hr half-life, that would be about six days.

If a drug checker suggests it takes 13 days for a substance with a 29hr half-life to clear your system, let's see how many half-life cycles it is suggesting is required:

13 days x 24hrs/1day = 312hrs x # half-life cycles/29hrs = 11 half-life cycles

Instead of the 11 half-life cycles the drug checker is suggesting it takes for this substance to clear one's system, Slow_Mobius is suggesting that for most of our purposes it only takes 5 half-life cycles. That is why he gives the 6 days figure instead of 11 days.
 
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There's a cutoff where you say enough-is-enough, 95% for a lot of probabilities, or 2 standard deviations for populations (95.4%, or "5 sigma" in physics which are odds of one in 3.5 million. Which you pick depends on how and what you measure.

And then, there's always the possibility that the particular site you're using just got lost in its code somewhere. The internet is rarely wrong, but it happens.


And folks, a cyp3A4 slow metabolism nearly killed me. It's why even if your site worked, OP, the 29-hour figure is going to be quite fuzzy in reality, possibly dramatically so. Just things to keep in mind.
 
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Ok so all i have to do is times the half life by 5 & that is when the drug will be out of my system? Didnt understand much of those last two posts....too many big words lol
 
Hopefully this is an easier way to explain it.

If you take the half life and multiply it by five, you will have eliminated 97% of the drug from your system by that time. Like we've also mentioned though, half lives can significantly vary between different people, so there's still no guarantee that that will be totally accurate in for you personally. We can't give you any answer with total certainty.

If anything still doesn't make sense, I can try to explain it again another way. Everyone thinks differently :\
 
I think like a freshman chemistry student with this stuff. I can totally relate to not being able to make sense of something, at least not at first :)
 
Scrofula said:
And folks, a cyp3A4 slow metabolism nearly killed me.
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Great point man. My friend who has since passed (FlyWithCloud9) was a hyper-metabolizer of certain drugs; i.e. he could take 600mg oxycodone and barely feel it. Doctors did a work up on his liver to prove this was real, it was. Loved him, I was so sad when he died.

Some others, like yourself, may be slow or inefficient metabolizers of certain enzymes.

Mr Dobalina said:
Ok so all i have to do is times the half life by 5 & that is when the drug will be out of my system? Didnt understand much of those last two posts....too many big words lol
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which drug are you taking?
 
Captain.Heroin said:
Great point man. My friend who has since passed (FlyWithCloud9) was a hyper-metabolizer of certain drugs; i.e. he could take 600mg oxycodone and barely feel it. Doctors did a work up on his liver to prove this was real, it was. Loved him, I was so sad when he died.

Some others, like yourself, may be slow or inefficient metabolizers of certain enzymes.



which drug are you taking?
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Im taking Lamictal
 
Question: so if I was taking for example an antidepressant and it took 6 days to clear my system, if I took a drug that antidepressants block on say the 4th day would I at least get some effects from the drug seeing as its been 4 days & most of the antidepressant is out of my system?

*edit* Today will be my 3rd day off Lamictal & I just took some Heroin. I feel a little bit warm & was alot more talkative to this lady I bumped into. So it does work just not to the full extent as if it were completely out of my system.
 
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