The Mayo Clinic stated in their Aug. 25, 2006 article "Marijuana as Medicine: Consider the Pros and Cons," published on its website:
"When smoked or ingested, THC and other cannabinoids in marijuana attach to two types of receptors on cells in your body — like keys in a lock — affecting the cells, once attached.
CB1 is one such receptor. CB1 receptors are found mainly in your brain, especially in areas that control body movement, memory and vomiting. This helps explain why marijuana use affects balance and coordination and impairs short-term memory and learning, and why it can be useful in treating nausea, pain and loss of appetite.
The other type of receptor, CB2, is found in small numbers elsewhere in your body, mainly in tissue of the immune system, such as your spleen and lymph nodes. The function of these receptors is not well understood. They may serve as brakes on immune system function, which may help explain why marijuana suppresses your immune system.
After you smoke marijuana, its ingredients reach their peak levels in your body within minutes, and effects can last up to an hour and a half. When eaten — the plant is sometimes mixed with food — the ingredients can take several hours to reach their peak levels in your body, and their effects may last for hours."
The US Drug Enforcement Administration (DEA) stated in its website article "Marijuana" (accessed Dec. 7, 2006):
"When marijuana is smoked, its effects begin immediately after the drug enters the brain and last from 1 to 3 hours. If marijuana is consumed in food or drink, the short-term effects begin more slowly, usually in 1/2 to 1 hour, and last longer, for as long as 4 hours. Smoking marijuana deposits several times more THC into the blood than does eating or drinking the drug.
Within a few minutes after inhaling marijuana smoke, an individual's heart begins beating more rapidly, the bronchial passages relax and become enlarged, and blood vessels in the eyes expand, making the eyes look red. The heart rate, normally 70 to 80 beats per minute, may increase by 20 to 50 beats per minute or, in some cases, even double. This effect can be greater if other drugs are taken with marijuana.
As THC enters the brain, it causes a user to feel euphoric — or 'high' — by acting in the brain's reward system, areas of the brain that respond to stimuli such as food and drink as well as most drugs of abuse. THC activates the reward system in the same way that nearly all drugs of abuse do, by stimulating brain cells to release the chemical dopamine.
A marijuana user may experience pleasant sensations, colors and sounds may seem more intense, and time appears to pass very slowly. The user's mouth feels dry, and he or she may suddenly become very hungry and thirsty. His or her hands may tremble and grow cold. The euphoria passes after awhile, and then the user may feel sleepy or depressed. Occasionally, marijuana use produces anxiety, fear, distrust, or panic."
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."