An interesting page from the University of Bristol: http://www.chemsoc.org/exemplarchem/entries/2004/bristol_rosling/My%20Webs/LSD.HTM "Lysergic acid diethylamide, more commonly known as LSD, is a non-toxic, non-addictive molecule which mimics serotonin in the brain. The body 'mistakes' LSD for serotonin and shoots it across the synaptic cleft."
that didnt give much of an indication at all though apparently lsd is in fact a catalyst for neurological change, not a direct instigator. concluded from the fact that lsd breaks down in an hour or two but the trip lasts hours
Well, however you slice it, that's still a lot of molecules floating around the ole' head. I figure (based on a heavily rounded 400g/mol of the tartaric acid salt), with a dose of around 200 mcg., that that's around 3x10^17 molecules. A trillion is 1x10^12, so were talkin' roughly 3 hundred million billion molecules. That's a bunch. If you have a hard time grasping that, stop taking stuff and get thee to Chemistry class.
Like I said, not all of the dose you eat makes it through the blood brain barrier. It's not important enough to me to look up the numbers, but the study is somewhere, and it's probably not that hard to find.