Oakley Ray, PhD, and Charles Ksir, PhD, noted in their 2004 textbook Drugs, Society and Human Behavior:

“When smoked, THC is rapidly absorbed into the blood and distributed to the rest of the body, so that within thirty minutes much is gone from the brain. The psychological and cardiovascular effects occur together, usually within five to ten minutes. The THC remaining in the blood has a half-life of about nineteen hours, but metabolites [any substances produced by biological processes](of which there are at least forty-five), primarily 11-hydroxy-delta-9-THC, are formed in the liver and have a half-life of fifty hours.

After one week, 25 to 30 percent of the THC and its metabolites might still remain in the body. Complete elimination of a large dose of THC and its metabolites might take two or three weeks.

THC taken orally is slowly absorbed, and the liver transforms it to 11-hydroxy-delta-9-THC; therefore, much less THC reaches the brain after oral ingestion, and it takes much longer for it to have psychological effects.

The high lipid solubility of THC means that it (like its metabolites) is selectively taken up and stored in fatty tissue to be released slowly. Excretion is primarily through the feces. All of this has two important implications:

  • there is no easy way to monitor (in urine or blood) THC/metabolite levels and relate them to behavioral and/or physiological effects, as can be done with alcohol, and
  • the long-lasting, steady, low concentration of THC and its metabolites on the brain and other organs might have effects not yet determined.”