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Dr. Bob is Robert Hsiung, MD,
bob@dr-bob.orgShown: posts 24 to 48 of 73. Go back in thread:
Posted by IsoM on April 5, 2002, at 15:22:47
In reply to ELIZABETH: what do AFAIk and HTH stand for?! (nm) » Elizabeth, posted by Janelle on April 5, 2002, at 14:49:07
Posted by fachad on April 5, 2002, at 15:23:54
In reply to FACHAD: what does GC/MS stand for? (nm) » fachad, posted by Janelle on April 5, 2002, at 14:46:52
GC/MS = Gas Chromatography / Mass Spectrometry
It's a very sensitive analytical technique that chemists use to determine what chemicals are present in a sample, and at what concentrations they are present.
Here's a few links if you want to see more, but its heavy chemistry type stuff, so be forewarned:
http://www.shsu.edu/~chemistry/primers/gcms.html
http://ull.chemistry.uakron.edu/gcms/
http://www.home-healthtesting.com/gcmstests.htm
Posted by IsoM on April 5, 2002, at 15:33:38
In reply to How can 1/2 life be SAME for DIFF concentrations?, posted by Janelle on April 5, 2002, at 14:45:06
It's not simply the measurement of a drug but but how much is needed to work. If 500 mg of drug X has the proper effect then at 5-7 half-lives, what's left of it would have little effect on the body & not even be noticed.
On the other hand, if only 5 mg of drug Y is needed for effectiveness, then with just one half-life, concentration would be at 2.5 mg - much less than 15.625 mg of drug X (now at 5 half-lives) still it's not the amount left but how much is needed to cause an effect.
Think of the difference in how much salt as compared to sugar is needed in a cake to be good tasting as a comparison.
Posted by JohnX2 on April 5, 2002, at 15:51:32
In reply to How can 1/2 life be SAME for DIFF concentrations?, posted by Janelle on April 5, 2002, at 14:45:06
> How can the number of half lives required to eliminate a drug (5-7 half lives and it's gone from body) be the SAME for DIFFERENT concentrations of the drug?
>
> Meaning, wouldn't a drug with a 100 mg/l concentration have fewer half-lives than one with say 350 mg/l concentration?
>
> I don't get this! Please help! Thanks!Its not *exactly* the same, but its *really miniscule* after many half lifes relative to the base dose, if you use a really simplistic model of exponential 1/2 life decay.
I'll take your example into account:
time ----dose---------
t1 350.0 100.0
t2 175.0 50.0
t3 87.5 25.0
t4 43.7 12.5
t5 21.8 6.2
t6 10.9 3.1
t7 5.4 1.5
t8 2.7 0.8John
Posted by fachad on April 5, 2002, at 15:51:45
In reply to FACHAD: you forgot my second question, here it is: » fachad, posted by Janelle on April 5, 2002, at 14:56:44
Janelle,
Your second question turns on your definition of "gone".
Take another look at my earlier post
http://www.dr-bob.org/babble/20020402/msgs/101982.html
In your question, you are assuming that less than 1 mg/l is "gone".
But remember that gone is a *relative* term, meaning that there is no longer a significant amount in the system, not an *absolute* term, meaning that there is absolutely none of it present at all.
For example, 1mg/l, what you were assuming meant "gone" could also be written as 1,000 mcg/l (micrograms per liter). You could watch it go from 1000 mcg/l to 500 mcg/l, and then from 250 mcg/l to 125 etc. At 1 mcg/l you are now at 1000 ng/l (nanograms per liter) and so on forever. If you have sensitive enough equipment, you could continue to measure decreasing concentrations long beyond when the drug was exerting any effects or causing any problems.
So, no matter what your starting concentration is, after 5 half-lives you are at 1/32 of where you started, which is *effectively* gone, no matter what the actual number is.
Hope this doesn’t' confuse you more.
Posted by fachad on April 5, 2002, at 16:12:06
In reply to How can 1/2 life be SAME for DIFF concentrations?, posted by Janelle on April 5, 2002, at 14:45:06
See my post above for my version of the answer:
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!
Posted by Janelle on April 5, 2002, at 17:10:39
In reply to What ^(-5) means » Janelle, posted by IsoM on April 5, 2002, at 15:19:30
How do you raise 2 to the negative 5?
Posted by Janelle on April 5, 2002, at 17:12:34
In reply to AFAIk/as far as I'm concerned HTH/hope that helps (nm) » Janelle, posted by IsoM on April 5, 2002, at 15:22:47
As Far As I'm CONCERNED - the word CONCERNED begins with a C, not a K! So why would it be AFAIk, not AFAIc ???? LOL???!!!???
Posted by Sunnely on April 5, 2002, at 17:19:32
In reply to Relationship btw 1/2 life and onset of action?, posted by Janelle on April 5, 2002, at 2:04:32
There is no relationship between half-life of a drug and its onset of action.
For drugs given orally, the rate of drug absorption from the gastrointestinal tract, for example, largely determines the speed of onset of action after a single dose. Drugs that are absorbed rapidly produce a faster and more intense onset of clinical effects, whereas the reverse is true for more slowlly absorbed drugs. No surprise that drugs given intravenously has the most rapid onset of action because it goes directly into the circulation.
The termination of a drug's effects after a single oral dose is largely determined by the rate and extent of the drug's distribution. For example, highly lipid-soluble (fat-soluble) drugs tend to be rapidly and extensively distributed throughout the body's tissues, which indicates a relatively brief duration of clinical effects following a single dose. Distribution, rather than elimination half-life of the drug, determines the duration of action after a single dose, but after repeated dosing, the elimination half-life becomes clinically important. For example, diazepam (Valium) has more rapid onset of action than lorazepam despite the former drug's longer half-life. This is due to diazepam's greater lipid solubility and more extensively distributed.
> I'm sorry to post this again, but I'm afraid it will get *lost* above because I mistakenly typed it in all caps and then a post appeared just below it in caps, making the two posts hard to distinguish.
>
> Here's my question:
>
> Is there a relationship between how long it takes a drug to start working (e.g. 2 hours after being taken) and that drug's half life?
>
> Meaning do drugs that take less time to start working also have shorter half-lives? Conversely, do drugs that take longer to start working also have longer half lives?
>
> thanks!
Posted by Janelle on April 5, 2002, at 17:34:14
In reply to Re: What does ^- mean in a computation? » Janelle, posted by JohnX2 on April 5, 2002, at 15:16:18
Posted by Janelle on April 5, 2002, at 17:48:48
In reply to Re: How can 1/2 life be SAME for DIFF concentrations? » Janelle, posted by IsoM on April 5, 2002, at 15:33:38
Okay, I get that if 500 mg of drug X has the proper effect then at 5-7 half-lives, what's left of it would have little effect on the body & not even be noticed. So, after 5 half-lives, 15.625 would basically have no effect. Heck, after 2-3 half-lives I would think there would not be enough to have a decent effect if 500 mg is needed?
Now, if only 5 mg of drug Y is needed for effectiveness, then with just one half-life, concentration would be at 2.5 mg, which I would think is too little to have an effect?
So, in a way, a med that requires a higher concentration (e.g., the one that requires 500 mg)needs more half-lives (than the one requiring only 5 mg) before it becomes ineffective, with 5-7 half-lives being the max? Is this accurate?ACKKKKKKKK!
I don't even get your comparison of the difference in how much salt as compared to sugar is needed in a cake to be good tasting! All I can think of is that you need a lot more sugar than salt for a cake to taste good, so you need a higher concentration of sugar than of salt.
Posted by Janelle on April 5, 2002, at 18:09:50
I've got info from two different threads that I've gotta combine here to ask my question so please bear with me. Thanks!
Okay, in one thread it said that no matter what your starting concentration is, after 5 half-lives you are at 1/32 of where you started, which is *effectively* gone.
Under another thread it said that after 5 half-lives there is 2^-5 = 1/32 of the original amount
left. Where does this equation come from? Why is it used - is it an equation that some genius came up with to realize that it takes 5 half lives to effectively eliminate a drug? HELP!
Posted by JohnX2 on April 5, 2002, at 18:27:55
In reply to WHY is 2^-5 used to compute max # half-lives?, posted by Janelle on April 5, 2002, at 18:09:50
> I've got info from two different threads that I've gotta combine here to ask my question so please bear with me. Thanks!
>
> Okay, in one thread it said that no matter what your starting concentration is, after 5 half-lives you are at 1/32 of where you started, which is *effectively* gone.
>
> Under another thread it said that after 5 half-lives there is 2^-5 = 1/32 of the original amount
> left. Where does this equation come from? Why is it used - is it an equation that some genius came up with to realize that it takes 5 half lives to effectively eliminate a drug? HELP!The definition of a 1/2 life is that at time t+1 there is 1/2 of the medicine that there was at time t.
so for 5 , 1/2 lifes:
we have:
1/2 * 1/2 * 1/2 * 1/2 * 1/2 = 1 / (32) = 1 / ( 2 ^ 5) = 2 ^ -5
No genious. Just math.
John
Posted by Janelle on April 5, 2002, at 19:00:43
In reply to Re: WHY is 2^-5 used to compute max # half-lives? » Janelle, posted by JohnX2 on April 5, 2002, at 18:27:55
Posted by JohnX2 on April 5, 2002, at 19:10:27
In reply to I get the math now, but why 5, it seems arbitrary (nm) » JohnX2, posted by Janelle on April 5, 2002, at 19:00:43
Posted by Janelle on April 5, 2002, at 19:10:51
In reply to Re: WHY is 2^-5 used to compute max # half-lives? » Janelle, posted by JohnX2 on April 5, 2002, at 18:27:55
Am I right in thinking that t + 1 refers to the time the drug was taken (t) and the 1 refers to one half life, so at t+1 50% of the drug would have been eliminated?
Posted by JohnX2 on April 5, 2002, at 19:13:12
In reply to Another (easy!) question re your explanation! » JohnX2, posted by Janelle on April 5, 2002, at 19:10:51
Posted by Janelle on April 5, 2002, at 19:16:48
In reply to Re: WHY is 2^-5 used to compute max # half-lives? » Janelle, posted by JohnX2 on April 5, 2002, at 18:27:55
It has been stated on here numerous places that it takes FIVE half-lives to effectively eliminate a drug from the body.
How did this number of FIVE half-lives to eliminate a drug become a *given* - did someone somehow figure out that it takes 5 half-lives to eliminate a drug?
Next, using 5 as a *given*, how could you plug 6, 7 or whatever number into your example of:
so for 5 , 1/2 lifes:
we have:
1/2 * 1/2 * 1/2 * 1/2 * 1/2 = 1 / (32) = 1 / ( 2 ^ 5) = 2 ^ -5If you did 1/2 *1/2 for 6,7,8, whatever number of times you would not get the 1/32 answer that is the amount of the drug left after 5 half-lives.
Ack, this is bugging me out!
Posted by JohnX2 on April 5, 2002, at 19:24:10
In reply to EE-GADS! No, it can't be 6, 7, etc. because: » JohnX2, posted by Janelle on April 5, 2002, at 19:16:48
I don't know who says that "1/32" is some magical barrier? Why not 1/64 , 1/128 ?For 1/32 you DO need exactly 5 half lives.
John
> It has been stated on here numerous places that it takes FIVE half-lives to effectively eliminate a drug from the body.
>
> How did this number of FIVE half-lives to eliminate a drug become a *given* - did someone somehow figure out that it takes 5 half-lives to eliminate a drug?
>
> Next, using 5 as a *given*, how could you plug 6, 7 or whatever number into your example of:
>
> so for 5 , 1/2 lifes:
>
> we have:
> 1/2 * 1/2 * 1/2 * 1/2 * 1/2 = 1 / (32) = 1 / ( 2 ^ 5) = 2 ^ -5
>
> If you did 1/2 *1/2 for 6,7,8, whatever number of times you would not get the 1/32 answer that is the amount of the drug left after 5 half-lives.
>
> Ack, this is bugging me out!
Posted by Janelle on April 5, 2002, at 19:46:44
Just who the heck (some famous pdoc) came up/invented the concept that it takes FIVE half-lives for any med to be effectively gone from a person's body????????
How was this number 5 derived????
Posted by Janelle on April 5, 2002, at 19:50:32
In reply to Re: EE-GADS! No, it can't be 6, 7, etc. because: » Janelle, posted by JohnX2 on April 5, 2002, at 19:24:10
You wrote "For 1/32 you DO need exactly 5 half lives."
How do you go from 1/32 to get the 5 (for number of half lives)??
Posted by JohnX2 on April 5, 2002, at 20:25:44
In reply to JohnX2: how does this work in reverse: » JohnX2, posted by Janelle on April 5, 2002, at 19:50:32
> You wrote "For 1/32 you DO need exactly 5 half lives."
>
> How do you go from 1/32 to get the 5 (for number of half lives)??log2(32) = 5
John
Posted by Janelle on April 5, 2002, at 20:28:22
In reply to Re: JohnX2: how does this work in reverse: » Janelle, posted by JohnX2 on April 5, 2002, at 20:25:44
Posted by Janelle on April 5, 2002, at 20:30:44
nfm
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