Half Life Of Fentanyl?

[Turing Machine]

I'm a bit confused by this. Wikipedia lists it as 3-12 hrs with 7 hours average while morphine is only 2-3 hours. I understand transdermal administration extends the effects of the drug even after the patch has been removed but I don't know if that would technically be considered an extension of the half life. Even if the half life is as short as three hours with other methods afministration this would still place it as longer acting than morphine which doesn't seem right to me. On the few occasions when I've received a fentanyl injection in the hospital for brief exploratory procedures it seemed very short lived. Does fentanyl indeed have an equal or longer half life than morphine, and I'm just incorrectly relating the length of the euphoric period with the half life? Assuming an equivelent dose, is it capable of keeping an addict "well" just as long as morphine but not high for as long? Or is it indeed a very short half life and wikipedia is wrong?

 

[dread]

Hmm. I do know half-life does not necessarily correlate with perceived length of effects.

One possible explanation could be that fentanyl produces tachyphylaxis, which would shorten the effects... not sure if this is the right answer though, just speculating.

 

[polarbearsarecool]

well half life does not all the time correlate to the time one feels a substance.. because many are metabolized into inactive metabolites etc..

 

[vecktor]

I have never seen data for cerebrospinal half life, the plasma half life is only part of the story as a lot of the stuff is protein bound and therefore unavailable for immediate CNS penetration.

I suspect the CNS half life is much shorter or there is much more rapid receptor down regulation than traditional opiates, the one piece of data that doesn't fit is that when naltrexone or similar opiate antagonists are used to treat fentanyl overdose they often have to be adminstered more than once because the fentanyl is still around, possibly it is being stored somewhere other than the cns.

fentanyl depots in the skin with transdermal patches so absorbtion continues after the patch is removed.

 

[El_Toro]

Also, with such fast acting, short lived drugs you need to take into account the acute downregulation of receptors which would also explain why half-lives do not correlate with perceived drug effect.

 

[kidamnesiac]

pretty sure I remember it rapidly partitions into fatty tissue and bound to protein, where it is slowly relased in equilibrium with the blood concentration.

the effects are much shorter than the half life leads one to believe.