I can't remember the exact figure from my research back in the seventies, but I believe several studies showed that only an infinitesimal fraction of the ingested LSD ever makes it as far as the brain. In fact, the highest concentration of it is found in the liver, the LOWEST concentration is found in the brain.
If I recall correctly, it is one ten thousandth, or maybe even only one hundred-thousandth of the dose that crosses the blood brain barrier. At any rate, I do remember that it is only a couple million molecules out of the standard 100 microgram dose used for clinical research at the time.