With all the hormones and whatnot inside. Dopamine, adrenaline, melatonin, whatever. Also, there’s this Hunter S. Thompson bit on the… pineal gland?
If it had an effect on you, wouldn’t it be a really messed up high, all over the place? With uppers, downers etc mixed? (Not including the emotional implications of eating raw human brains.)
The lower colon really only transports water and a couple of vitamins released by gut biota, so if they’re getting far enough in that more complex molecules would be taken up, that would probably be up into the hepatic portal system.
The reason “boofing” works for alcohol is because ethanol is actually a rather small molecule, all things considered.