Think of heroin not so much as a different drug than morphine but as 2 acetyl groups which act as a vehicle for the faster and more efficient transmission of morphine across the blood-brain barrier. Non-parenteral administration of morphine and diamorphine produces identical effects, IV heroin produces a bigger rush because it crosses the BBB faster.
Well put... a lot of ppl even experienced users are unaware of exactly how short the half-life of heroin as its own molecule is. It is LITERALLY like less than 5 minutes, no more than 5 min.. but by then the acetyl group (which actually hinders heroin's affinity for the mu-receptor!) did its job of getting its precious cargo across the blood-brain barrier.
Of course shortly thereafter one of these "vehicle" acetyl groups is removed in vivo to create either 6-
monoacetylmorphine (6-MAM) or 3-MAM. This is in contrast to another common misconception... that the heroin (diacetylmorphine) is actually converted straight to morphine directly when metabolized.
I am fairly sure that the"straight to morphine" process does not occur at all... and if it does, it happens on a very small, negligible scale in the grand scheme of things.
In fact, only ONE acetyl group ("vehicle") is removed at a time during heroin's metabolism... 6-MAM being the more active monoacetylmorphine over 3-MAM in terms of mu-receptor affinity/analgesic effect.
As has been pointed out, 6-monoacetylmorphine has a higher mu-receptor affinity than both morphine and heroin (3-MAM as well). 6-MAM has a much longer, more opiate-typical half-life than heroin and plays an underrated role in heroin's euphoric advantage over morphine.
The 6-MAM has the benefit of still having a "vehicle" acetyl group to help cross the BBB a lot faster/easier than plain morphine... AND (unlike heroin), having now lost one of the acetyl groups after the initial metabolizing, can produce the pharmacological action on opiate receptors that heroin cannot.
I've always wondered what taking pure 6-MAM would be like. I guess from the info I've read about it, it would basically be like a "super-morphine" much like what heroin could be considered, although to a somewhat lesser extent. Both of the enhanced versions of morphine have that key acetyl group "vehicle" to speed up the crossing the BBB and cause the drugs to work quicker than plain morphine. Of course, having TWO acetyl vehicles ensures heroin's faster action. So I bet 6-MAM would probably feel like a bit of a slower acting heroin, of course having similarities to morphine.
Lastly, and I never really hear this mentioned in such discussions, but aside from the aforementioned advantages over morphine I think that the simple fact of the heroin molecule having to go through all the processes it does once in the body has GOT TO play a factor in the length of the high, in a good way of course.
When you think about it, heroin's modest advantage over morphine of inducing a high with a little longer duration (the quicker rush is a FAR bigger factor popularity-wise) should pretty much be considered by default... the changes heroin undergos in its metabolic process (as mentioned last paragraph) make the prodrug into to mini-cocktail of sorts containing some active metabolites with different half-lives and qualities.
***There is a lot to be said for fact that morphine can actually be considered just another stop (albeit a significant one) during the journey of the heroin molecule's metabolizing in the body. And a lesser one at that, somewhat further into the unfolding of the process than the superior 6-MAM. This fact, plus heroin's more favorable side-effect profile, makes DIACETYLmorphine (yes, that's heroin) unequivocally superior to morphine, IMHO***
Wow, sorry that got a bit long there... hope someone learned something!! I'm sure a lot of ya'll already knew a lot of this but I liked the "vehicle" analogy... and I'm bored.
