Re: Relationship btw 1/2 life and onset of action? » Janelle

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!

Thread

Could someone explain HALF-LIFE w/ example! Janelle 4/4/02
...
What does ^- mean in a computation? Janelle 4/5/02
Re: What does ^- mean in a computation? » Janelle JohnX2 4/5/02
What ^(-5) means » Janelle IsoM 4/5/02
AFAIk/as far as I'm concerned HTH/hope that helps (nm) » Janelle IsoM 4/5/02
GC/MS = Gas Chromatography / Mass Spectrometry » Janelle fachad 4/5/02
Re: How can 1/2 life be SAME for DIFF concentrations? » Janelle IsoM 4/5/02
Re: How can 1/2 life be SAME for DIFF concentrations? » Janelle JohnX2 4/5/02
2nd Question- It's All Relative » Janelle fachad 4/5/02
See Above Post » Janelle fachad 4/5/02
Re: FACHAD: 2 questions 4 U (1/2 lives revisited!) » Janelle Sunnely 4/5/02
ISO: thx, how do you raise to a negative number: Janelle 4/5/02
ISO: you are GREAT! But shouldn't it be AFAIc for: Janelle 4/5/02
Re: Relationship btw 1/2 life and onset of action? » Janelle Sunnely 4/5/02
No need 4 redirect, I understand! Thanks, JX2! (nm) » JohnX2 Janelle 4/5/02
ISO: Ack, I'm having trouble following that » IsoM Janelle 4/5/02
WHY is 2^-5 used to compute max # half-lives? Janelle 4/5/02
Re: WHY is 2^-5 used to compute max # half-lives? » Janelle JohnX2 4/5/02
I get the math now, but why 5, it seems arbitrary (nm) » JohnX2 Janelle 4/5/02
yes,5 is arbitrary, could just as well be 6 or 7.. (nm) » Janelle JohnX2 4/5/02
Another (easy!) question re your explanation! » JohnX2 Janelle 4/5/02
Re: Another (easy!) question - Bingo! (nm) JohnX2 4/5/02
EE-GADS! No, it can't be 6, 7, etc. because: » JohnX2 Janelle 4/5/02
Re: EE-GADS! No, it can't be 6, 7, etc. because: » Janelle JohnX2 4/5/02
1/2 LIFERS:did a famous pdoc invent the following: Janelle 4/5/02
JohnX2: how does this work in reverse: » JohnX2 Janelle 4/5/02
...
Re: Adam, I have no idea what you did! what is ''e''? 2sense 4/15/02

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