When marijuana is smoked, its effects begin immediately after the drug enters the brain and last from 1 to 3 hours.

Smoking marijuana deposits several times more THC into the blood than does eating or drinking the drug.

And it says this, at the bottom:
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Glossary

Addiction: A chronic, relapsing disease characterized by compulsive drug-seeking and abuse and by long-lasting chemical changes in the brain.

Cannabinoids: Chemicals that help control mental and physical processes when produced naturally by the body and that produce intoxication and other effects when absorbed from marijuana.

Carcinogen: Any substance that causes cancer.

Dopamine: A brain chemical, classified as a neurotransmitter, found in regions of the brain that regulate movement, emotion, motivation, and pleasure.

Hippocampus: An area of the brain crucial for learning and memory.

Hydrocarbon: Any chemical compound containing only hydrogen and carbon.

Psychoactive: Having a specific effect on the mind.

THC: Delta-9-tetrahydrocannabinol; the main active ingredient in marijuana, which acts on the brain to produce its effects.

Withdrawal: Symptoms that occur after use of a drug is reduced or stopped.


But, look at the references from this study.

As people age, they normally lose neurons in the hippocampus, which decreases their ability to remember events. Chronic THC exposure may hasten the age-related loss of hippocampal neurons. In one series of studies, rats exposed to THC every day for 8 months (approximately 30 percent of their lifespan), when examined at 11 to 12 months of age, showed nerve cell loss equivalent to that of unexposed animals twice their age.
References
67Landfield, P.W.; Cadwallader, L.B.; and Vinsant, S. Quantitative changes in hippocampal structure following long exposure to delta-9-tetrahydrocannabinol: Possible mediation of glucocorticoid systems. Brain Res 443(1-2):47-62, 1988.

68Eldridge, J.C.; Murphy, L.L.; and Landfield, P.W. Cannabinoids and the hippocampal glucocorticoid receptor: recent findings and possible significance. Steroids 56:226-230, 1991.

69Landfield, P.W.; Waymire, J.C.; and Lynch, G. Hippocampal aging and adrenocorticoids: quantitative correlations. Science 202:1098-1101, 1978.
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