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Dr. Bob is Robert Hsiung, MD,
bob@dr-bob.orgShown: posts 1 to 12 of 12. This is the beginning of the thread.
Posted by flip_flop on May 13, 2006, at 1:51:10
Let's say I take 100 mg of some drug that has a half-life of 6 hours. Does this mean that after 6 hours, 50 mg of the drug is still active? This drug would need to be taken at least 2 times/day, but why couldn't I just take twice the dosage only once/day?
Posted by Racer on May 13, 2006, at 2:16:28
In reply to What does half-life really mean?, posted by flip_flop on May 13, 2006, at 1:51:10
It's the length of time for half the drug to be metabolized out of your system, which is not really the same thing. Someone else will have to answer more thoroughly, I'm half asleep already tonight and my brain seems sluggish...
Posted by Meri-Tuuli on May 13, 2006, at 7:21:53
In reply to What does half-life really mean?, posted by flip_flop on May 13, 2006, at 1:51:10
Hi!
> Let's say I take 100 mg of some drug that has a half-life of 6 hours. Does this mean that after 6 hours, 50 mg of the drug is still active?
Yes. Half the drug would have been converted/metabolised by your body, and the other half still there.
>This drug would need to be taken at least 2 times/day, but why couldn't I just take twice the dosage only once/day?
With drugs that have a short half life, then they will enter your bloodstream (and therefore be used) quickly. Then the level will fall as your body is using up the med. Thing is, you need the twice (or whatever) daily dosing to try and keep the levels stable-ish in your bloodstream, so you don't get big peaks (too much medication) and big troughs (too little). This might cause unwanted side effects, and make you not feel very well for instance. So you need to spead the dosage out during the day.
If the med has a long half life (like prozac for instance) then once daily dosing is fine.
Posted by Larry Hoover on May 13, 2006, at 9:10:50
In reply to What does half-life really mean?, posted by flip_flop on May 13, 2006, at 1:51:10
> Let's say I take 100 mg of some drug that has a half-life of 6 hours. Does this mean that after 6 hours, 50 mg of the drug is still active? This drug would need to be taken at least 2 times/day, but why couldn't I just take twice the dosage only once/day?
The target you're shooting for is stable blood concentration. Your last example would be an extreme roller coaster. Ideally, you dose at half the intended blood concentration, with a frequency equal to the half life. Let's use your numbers, and work out the math.
Time zero: First dose, 100 mg.
Time +6: 50 mg remaining from first dose, plus 100 mg more = 150 mg. Symbolically, you now have in your blood 1 + 1/2 doses.
Time +12: 75 mg remaining from before, 25 from Tzero, 50 from T6, plus 100 mg more. You now have 1 + 1/2 + 1/4 doses in your blood.
Time +18: 87.5 mg remaining from before, 12.5 from Tzero, 25 from T6, and 50 from T12, plus 100 mg more. You now have 1 + 1/2 + 1/4 + 1/8 doses in your blood.
T24: 93.75 remaining from before.....1 + 1/2 + 1/4 + 1/8 + 1/16The limiting value of the first series is 1 (dose), and that of the second series is 2. I.e. the sum of those fractions that make up each series approaches infinitely close to the numbers 1 and 2. So, a doctor sets the dose at half the expected peak plasma concentration, and sets the dosing frequency so that the interval between doses is the half-life of the drug. Blood concentrations oscillate between 1 and 2 times the dose, with a period of the half life.
It is generally assumed that five half-lives are required to reach fairly stable blood levels, but there are a lot of assumptions being made, and I don't want to go into those too much, yet. Not all drugs work by half-lives. Alcohol is metabolized at a fixed rate, independent of any concept such as half-life, for example (zeroth order rate law).
This example, as hypothesized, would be true for well over 90% of the drugs commonly used. There are some important exceptions, and even doctors screw up on those exceptions.
I hope this helps your understanding. Feel free to ask more questions.
Lar
Posted by Phillipa on May 13, 2006, at 12:14:15
In reply to Re: What does half-life really mean? » flip_flop, posted by Larry Hoover on May 13, 2006, at 9:10:50
Lar you're above me head. If a med is prescirbed twice a day I take it twice a day. And that is why it usually takes at least 5 days for the drug levels to build up to a theraputic level in your blood. No? Not surprised. Love Phillipa
Posted by Larry Hoover on May 13, 2006, at 15:58:44
In reply to Re: What does half-life really mean?, posted by Phillipa on May 13, 2006, at 12:14:15
> Lar you're above me head.
Well, on a map I am. Canada is up there, I suppose. I really had hoped to not be over people's heads with what I said.
> If a med is prescirbed twice a day I take it twice a day.
Usually, so do I (take meds as prescribed).
> And that is why it usually takes at least 5 days for the drug levels to build up to a theraputic level in your blood. No?
Five half lives, is considered standard. But, it's really an arbitrary standard. And the effect of the drug could have been immediate, or delayed, notwithstanding therapeutic concentration. There are so many factors....
> Not surprised. Love Phillipa
Not surprised about what? You lost me, there.
Lar
Posted by Phillipa on May 13, 2006, at 19:26:28
In reply to Re: What does half-life really mean?, posted by Phillipa on May 13, 2006, at 12:14:15
Lar I meant that I wouldn't be surprised to find I had it wrong which I did. Love Phillipa
Posted by flip_flop on May 14, 2006, at 2:17:21
In reply to Re: What does half-life really mean? » flip_flop, posted by Larry Hoover on May 13, 2006, at 9:10:50
OK, but is it always necessary to maintain stable blood concentrations of a drug? Seroquel, for example, is often taken in low doses for other disorders than schizophrenia. It has a short half-life and has to be taken several times/day. Unfortunately it's very sedating, which prevents some people from taking it in the daytime. But what if someone took a large enough dosage only at bedtime? This way the sedation might wear off, and the drug would be effective throughout the next day. Maybe it's not necessary to maintain stable concentrations of Seroquel when taken for other disorders than schizophrenia?
Posted by Larry Hoover on May 14, 2006, at 11:10:26
In reply to Re: What does half-life really mean? » Larry Hoover, posted by flip_flop on May 14, 2006, at 2:17:21
> OK, but is it always necessary to maintain stable blood concentrations of a drug? Seroquel, for example, is often taken in low doses for other disorders than schizophrenia. It has a short half-life and has to be taken several times/day. Unfortunately it's very sedating, which prevents some people from taking it in the daytime. But what if someone took a large enough dosage only at bedtime? This way the sedation might wear off, and the drug would be effective throughout the next day. Maybe it's not necessary to maintain stable concentrations of Seroquel when taken for other disorders than schizophrenia?
Under the circumstances you describe, e.g. employing Seroquel only for insomnia, then you are right on the money with your understanding. You have to understand the pharmacology of the drug, and then apply it to a specific situation.
The only unknown in your speculation is explicit knowledge of the drug's half-life in your unique body. You can't assume that the drug will have the same half-life as that found by experimentation during the drug development period. The published half-life is a statistical average. What is often missing from the published literature is the standard deviation for the mean half-life of the drug, or evidence for skewness.
Lar
Posted by Larry Hoover on May 14, 2006, at 11:12:50
In reply to Re: What does half-life really mean? » Phillipa, posted by Phillipa on May 13, 2006, at 19:26:28
> Lar I meant that I wouldn't be surprised to find I had it wrong which I did. Love Phillipa
You didn't have it wrong. In the case of a drug with a 24-hour half-life, 5 days is the accepted period over which blood concentration stabilizes. You were generalizing from one common scenario, which you had correct.
Lar
Posted by flip_flop on May 14, 2006, at 11:26:00
In reply to Re: What does half-life really mean? » flip_flop, posted by Larry Hoover on May 14, 2006, at 11:10:26
How about disorders like anxiety or OCD? Would a single dosage of Seroquel be adequate for that?
Posted by yxibow on May 16, 2006, at 3:59:31
In reply to Re: What does half-life really mean? » Larry Hoover, posted by flip_flop on May 14, 2006, at 11:26:00
> How about disorders like anxiety or OCD? Would a single dosage of Seroquel be adequate for that?
Single dosing of Seroquel is common in general, because it is positively the most sedating atypical antipsychotic (I know from experience, from its heavy load at H1) and perhaps the most sedating in general. It seems to work for Schizophreniform disorders.
I take it for a complex anxiety disorder, now nearly once a day. I am doing an experiment and taking 100 at 8pm and the remaining 900 at bedtime to increase the evening serum level, but in general, its regarded that the serum levels are adequate for once a day dosing.
Some people still may fall out of this category and still need twice or three times a day, and a lot of coffee or something, which I can't take.
-- tidings
Jay
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